What's going on when roots turn green and grow into the air?

What's going on when roots turn green and grow into the air?

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You're looking at a hibiscus and an orchid growing in the same pot. I'm curious why the hibiscus started growing green roots into the air. Or am I looking at a third plant that somehow made its way into the pot?

July 27th

All it does is branch and get longer.

There are no leaves and the shoot doesn't thicken or turn to bark.

A new sprout appears every few days.

Is the hibiscus so root bound it's trying to escape the pot?

July 9th

I found this article online that does describes hibiscus plants as having a "spreading" type root system that will also form new shoots away from the parent plant…

I also noticed that you have an attached orchid with aerial roots attached to the hibiscus plant. If you are misting the orchid roots periodically, you may be dampening the surface soil in the pot and encouraging the growth of shoot forming spreading roots on the hibiscus plant to develop and grow upward.

Orchid Roots: Complete Care Guide

Job number one is to absorb nutrients and minerals as well as water to feed and hydrate the plant. However, the way an orchid’s roots do this is somewhat different from what you are probably used to – they absorb these nutrients from the air.

2. Keep the plant safe and steady.

Job number two is to secure the plant in place. Although here again, when the roots of an orchid start growing every which way, it is easy to get confused!

3. Storing away extra nutrients.

Job number three is to become a food storage cellar, packing away extra nutrients to last during lean times.

If the orchid air roots are firm and white, they are healthy and you don’t need to do anything at all. Just accept that this is normal behavior. According to orchid experts, you should definitely not remove the roots. There’s a good chance you’ll harm the plant or introduce a dangerous virus.

Trim an orchid root or stem only if it’s dry and you’re certain it’s dead, but work carefully to avoid cutting too deep and harming the plant. Be sure to sanitize your cutting tool by wiping the blades with rubbing alcohol or a solution of water and bleach before you begin.

This may be a good time to check the size of the pot. If the plant seems a little too snug, move the orchid into a larger container because overcrowded roots may escape and look for space to grow above the surface of the soil. Be sure to use a potting mix suitable for orchids. (Some orchid pros think that a perlite/peat mix is less likely to produce aerial roots than bark.) Either way, don’t cover the roots because they may rot.

The Root of Plants (With Diagrams)| Botany

The root is the descending axis of the plant and the down­ward prolongation of the radicle. It is usually non-green in colour and grows away from light. The root has no nodes and internodes. It bears only similar mem­bers, i.e. only roots.

The main root developing from the radicle is called the pri­mary root which, in turn, produces many secondary and tertiary branches, thus forming a system of roots going inside the soil in different directions.

These branch roots develop from internal tissues of the pri­mary root, so they are endo­genous in origin. They are arranged in acropetal suc­cession, i.e. older roots near the base and smaller and smaller ones towards the tip.

Regions of the Root (Fig. 32):

A root has usually four regions, in order, from the apex toward the Fig. 32.

(i) Root Cap Region:

The extreme tip of the root is usually protected by a cap or thimble-shaped body called the root-cap. When the root pushes its way through the soil, the tender and soft tip runs the risk of being injured due to friction with soil particles.

The root-cap, serving as a buffer, protects the tip from that danger. Further, the cap secretes slimy mucilaginous matters which facilitate the course of the root through the soil. The root-cap is absent in many aquatic plants. In screw pine root- caps are multiple (Fig. 33).

(ii) Region of Active Growth and Elongation:

Next to the root-cap is the region of active growth. It is a very short region where cells divide actively. This merges into the region of elon­gation where the newly formed cells grow in length.

This region is found next and here the outer cells of the roots grow out as unicellular hairs. The uni­cellular root-hairs are responsible for the absorption of water and mineral matters dissolved in water from the soil.

By germinating mustard seeds on a piece of soaked blotting paper the dense root-hairs can be conveniently observed (Fig. 34). During the growth of the root old zone is exhausted and is replaced by new zones of root-hairs more and more towards the tip.

This is located higher up. Here secondary roots develop from the primary root. The branches also have four regions like the primary root.

Root Systems (Fig. 35):

In dicotyledonous plants the radicle produces the primary root which is quite prominent, that is, long and stout. This is called the tap root. The tap root bears many secondary and tertiary branches developing acropetally.

All these branches can be easily distinguished from the main root. The whole system is called the tap root system. In monocotyledons like maize and rice the radicle produces the primary root which soon aborts and is re­placed by a tuft of roots developing from the base of the stem. All of them are more or less similar and are called fibrous roots and the whole system is known as fibrous root system.

Normal and Adventitious Roots:

Roots that develop from the radicle (primary root) or branches of the roots from that origin, are called normal or true roots and roots growing from any position other than the normal point of origin, are called adventitious roots. So the tap root system, usually found in the dicotyledons, is normal and fibrous root system, common in the monocotyledons like grasses, is adventitious.

Normal Functions of the Root:

Roots perform two normal functions. First, they fix the plants to the soil and thus secure anchorage. Secondly, roots are responsible for the absorption of water and dissolved mineral matters from the soil through the delicate root-hairs. Water is ultimately conducted to the leaves.

The primary roots with their large number of secondary and tertiary branches in the dicoty­ledons, or the fibrous roots in the monocotyledons penetrate into the soil, spread out in different directions and thus effectively carry on the normal functions.

In common land plants the root system, in fact, equals to or even exceeds the aerial portions (shoot system) both in length and volume. The fixation or anchorage is the mechanical function of the root and absorption and conduction of water and dissolved mineral matters, are the chief physiological functions.

Besides these, roots may perform many special functions which will be discussed in connection with the modified roots.

Modifications of Roots:

Many roots undergo modifications for carrying on functions other than the normal ones. Some of the modified roots are normal, while many of them are adventitious.

1. Normal Roots Modified for Storage of Food:

Some roots become swollen and fleshy due to the storage of food matters and assume different shapes (Fig. 36):

Here the root with the hypocotyl becomes swollen. The swelling is maxi­mum in the middle. It gradually tapers to­wards the ends, i.e. base and apex, e.g. radish. Practically the whole swollen portion in radish is the hypocotyl.

Here also the root is fleshy it is broadest at the base and gradually tapers towards the apex like a cone, e.g. carrot.

It is very much swollen at the base (with the hypocotyl), but abruptly tapers towards the apex, as in beet and turnip.

2. Adventitious Roots Modified for Storage of Food:

In plants like sweet potato which grow on the surface of the soil, some of the ad­ventitious roots, deve­loping from the stem, become swollen and fleshy due to the storage of food. They do not have any re­gular shape. These are called tuberous root (Fig. 37).

(e) In Asparagus (B. Satamuli), Ruellia, etc., quite a good number of tuberous roots are produced from the stem, form­ing a bundle or fasci­cle. They are called fasciculated roots (Fig. 38).

(f) In mango-ginger (B. Amada) the adventitious roots sud­denly become swollen at the tips. They are called nodulose roots (Fig. 39).

(g) In many grasses the swellings are often found at frequent intervals giving the root a beaded appearance. These peculiar roots are called moniliform (Fig. 39).

Adventitious Roots Modified:

Roots are normally underground organs. Some roots develop completely above ground and, so, are aerial. In the common banyan trees of our country roots are often found to hang freely in the air from the branches.

In course of time they reach the soil, become stouter and serve as so many extra supports or props. Thus they help the stem in bearing the weight of the heavy crown. These are called the prop roots (Fig.40).

These are also supporting roots. In plants like screw pine (B. Keya) quite a good number of adventitious roots develop from the basal part of the stem and go down­wards obliquely. These roots help the plant in maintaining the upright position. These are stilt roots (Fig. 41).

Weak climbing plants like betel, vine, Scindapsus (B. Gaj-pipul), etc., produce some adventitious roots which cling to the sup­ports and thus help the plants in climbing. They often sec­rete sticky juices or develop disc-like structures for that purpose (Fig. 42).

Many epiphytes, like orchids, pro­duce long roots which freely hang in air. They have sponge-like tissues, called velamen, at the ex­terior by means of which they can absorb moisture from the air (Fig. 43).

5. Sucking Roots or Haustoria:

Sucking roots or haustoria develop in the parasitic plants like Cuscuta or dodder (B. Swarnalata). These roots penetrate into the body of the host and draw nourishment from there without caring to manufacture their own food (Fig. 44, 2 & 3).

These roots are produced by the plants like Heritiera (B. Sundri), Rhizophora (B. Bora) growing in saline marshes or on the sea­shore where the soil is very poor in oxygen. Here some roots with pores at the tips come vertically surface of the soil and carry on the gaseous inter- with the change outer atmosphere. They are also called pneumatophores (Fig. 44—1).

In an aquatic plant fuissiaea (B. Keshar, dam) some peculiar, adventi­tious roots develop from the floating branches. They are light, soft and spongy due to the presence .of air­spaces. Besides faci­litating respiration, they give the plants buoyancy for float­ing on the surface of water (Fig. 45).

Roots of plants like Tinospora (B. Gulancha) hang freely in the air and. develop green colour. They can manu­facture food matters. The sub-merged roots of Trapanatans (water chest nut) coming out in pairs from the nodes are green and assimilatory in function.

Roots are also used for vegetative multiplication. Common vegetable plants like Trichosanthes dioica(B. Patol), sweet potato, etc., are propagated by means of roots. Many garden plants are also multiplied by root cuttings.

Plants could fight climate change by growing bigger roots

Plants and algae around the planet absorb some vast amounts of CO2 from the air each year, yet there’s a natural limit to how much they can do so as to reduce the amount of excess CO2 still in the atmosphere.

Science can help out, or so many scientists believe. Last year a team of biochemists at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany, succeeded in re-engineering the way plants, algae and various micro-organisms turn CO2 into fuel by making the process around 25% more energy efficient and significantly faster.

Now another team of scientists from the Salk Institute for Biological Studies in the United States is working on another way to harness plants’ carbon-absorbing natural abilities: this time their ability to capture carbon from the air and store it underground. Plants do that in their root systems so the larger those systems are and the deeper they go, the more CO2 plants can potentially lock up and store, explain the scientists in a study published in the journal Cell.

To understand the process of root formation, the experts examined the genetic and molecular mechanisms involved. In the process, they have discovered a gene that influences whether a plant’s roots grow deep down into the earth or remain shallow in the soil.

By manipulating that gene, we could cause cultivated plants to develop more robust and deeper roots, which would enable them to store larger amounts of carbon underground.

The researchers focused on a plant called thale cress (Arabidopsis thaliana). “In order to better view the root growth, I developed and optimized a novel method for studying plant root systems in soil,” explains the study’s first author Takehiko Ogura, a postdoctoral fellow. “The roots of A. thaliana are incredibly small so they are not easily visible, but by slicing the plant in half we could better observe and measure the root distributions in the soil.”

In the plant one gene called EXOCYST70A3, Ogura and his colleagues discovered, is responsible for regulating root system architecture. When they tampered with this gene in the lap, the scientists found plants started growing larger roots that went deeper into the soil.

Their work isn’t done yet, however. It’s one thing to do this in the lab and another to do it on a large-enough scale across cultivated farmlands so as to reduce the levels of CO2 in the atmosphere. Still, the hope is that we might be able to boost cultivated plants’ abilities in storing carbon in order to help us fight climate change.

“We hope to use this knowledge of the auxin pathway as a way to uncover more components that are related to these genes and their effect on root system architecture,” says Wolfgang Busch, an associate professor at Salk’s Plant Molecular and Cellular Biology Laboratory.

“This will help us create better, more adaptable crop plants, such as soybean and corn, that farmers can grow to produce more food for a growing world population,” he adds.

Why is the Nitrogen Cycle Important

As said before, nitrogen is important because it is one of the main building blocks of all life. Nitrogen is found in all living organisms like amino acids, which in turn make up proteins, nucleic acids, adenosine triphosphate, and many others. Actually, nitrogen is the basis of many parts of DNA including thymine, cytosine, adenine, and guanine. In RNA, the nitrogenous bases are the same with the exception of uracil, which is not present in DNA.


Simply, we could not exist without nitrogen. Since it makes up much of our atmosphere, is the base of many parts of DNA and RNA, life would unravel and nothing would be able to survive.

On some orchids, it can be tricky to tell the difference between a flower spike and an aerial root. This is how to tell the difference: an aerial root will have a smooth tip. The end of an emerging flower spike will look like a closed fist with knuckle bumps.

To help you further, start by downloading my free cheat sheet to see where to cut the orchid flower spike after blooms have faded to trigger re-blooming. Click here, for the cheat sheet. It’ll be super helpful.

The smooth tip is a sure sign that this is a root, not a flower stalk.

Pictured above is a flower stalk. If you see a “fist” at the tip, you know you’ve got a flower stalk.

Is it Okay to Cut off Aerial Roots?

It may be tempting to cut off aerial roots because they don’t fit into our idea of what a beautiful orchid should look like. Resist. Aerial roots perform important functions for the orchid. Like the leaves and stems, aerial roots aid in photosynthesis. Aerial roots also absorb water and nutrients in the air. Additionally, aerial roots aid the orchid in affixing itself to its host.

Watering and Aerial Roots

Sometimes aerial roots can look dried out and it can be tempting to water the orchid. To avoid rotting the roots down in the potting media, don’t let the aerial roots tell you when to water your orchid. Instead, look at, or even touch, the potting media. If it is still damp, wait to water.

If the aerial roots are looking dried out, you have several options. First, you can mist the roots with a spray bottle. Just be careful not to raise humidity levels above 50% in a home environment. Second, you could increase the humidity around your orchid. Third, just let them be.

Personally, I’m more inclined to go with the second and third options. The first one is too time-consuming and I find that if I’m too fussy with my orchids, I tend to cause more hard than good. I keep a cool-mist humidifier on the lowest setting. And, I just let those aerial do what they are going to do.

Potting Aerial Roots

When potting an orchid, leave aerial roots in the air and potted roots in the potting medium. Aerial roots have a thicker coating of velamin and physiologically different than roots that are growing in the potting medium.

One of the most important things your can do when your AeroGarden is just starting is to trim your plants. There are far too many types of plants that you can grow in your AeroGarden to discuss them all here…so we will only give you a few basic pointers that apply to most all plants. We highly recommend you follow the trimming instructions in the guide included in your AeroGarden Seed Kits. Here are some basic trimming principles you can use:

NEVER trim more than 1/3 of your plants at one time.

Once you trim, give them some time to continue growing and regenerate

If the plants would normally grow too tall for the AeroGarden, you can ‘train’ the plants to grow out instead of up by cutting the top of the plant off, above the 5th stem. This will encourage the plant to grow and fill out sideways rather than up.

Steps in Seed Germination – The Primary Phase of Plant Growth

A botanical seed consists of an embryonic plant that is in resting form. Seed germination is the basic phase in the growth of any plant.

A botanical seed consists of an embryonic plant that is in resting form. Seed germination is the basic phase in the growth of any plant.

Did You Know?

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Life cycle of any plant is divided into different phases and seed germination is basic stage of the growth of any plant. A seed contains the essence of a plant in a resting, embryonic condition. Whenever a seed gets a favorable environment, the stages of germination start taking place. A dormant seed lying in the ground needs warmth, oxygen, and water to develop into a plant.

Seed Structure

The seed coat is the outer covering of a seed which protects the embryo from any kind of damage, caused by the natural elements or due to the invasion of parasites, and prevents it from drying. A seed coat may be thick and hard, or thin and soft. The endosperm inside the seed coat contains a temporary nutritional reserve, which is packed around embryo in the form of cotyledons or seed leaves. Plants are classified as monocots and dicots depending upon number of cotyledons their seeds contain. Monocots are angiosperm seeds that contain a single seed-leaf or cotyledon, whereas, dicots are angiosperm seeds that contain two cotyledons.

Required Conditions

All seeds need adequate oxygen, water, and the right temperature for germination. Some seeds also need proper lighting. Some can germinate well in presence of light, while others may require darkness to start germination. Water is necessary for initiation and maintenance of an appropriate rate of metabolism. Soil temperature is equally important for appropriate germination. Optimum soil temperature for each seed varies from species to species.

Steps Involved

  1. The seed absorbs water and seed coat bursts. It is the first sign of germination. There is an activation of enzymes, increase in respiration, and plant cells get duplicated. A chain of chemical changes starts which leads to the development of the plant embryo.
  2. Chemical energy stored in the form of starch is converted to sugar, which serves as food for the embryo during the germination process. Soon, the embryo gets nourished and enlarged, and the seed coat bursts open.
  3. The growing plant emerges out. Tip of the root first emerges, growing downwards, and helps to anchor the seed in place. It also allows the embryo to absorb minerals and water from soil.
  4. Some seeds require special treatment of temperature, light or moisture to start germination.

Steps in seed germination can be different in dicots and monocots.

Germination in Dicots

During germination process of dicots, primary root emerges through the seed coat when seed is buried in soil. The hypocotyledonous stem (hypocotyl) emerges from seed coat and grows upwards through soil. As it grows up, it takes the shape of a hairpin, which is known as hypocotyl arch. Epicotyl structures, known as the plumule, are protected by two cotyledons from any kind of damage. When hypocotyl arch emerges out of the soil, it continues growing straight, the direction of growth being influenced by the direction of light. Cotyledons spread apart, exposing the epicotyl, which contains two primary leaves and the apical meristem. In many dicots, cotyledons supply nutritional matter to the developing plant and they also turn green (owing to the to the production of chlorophyll), which enable the plant to acquire nutrition via photosynthesis.

Germination in Monocots

During germination of seeds such as oats or corn, the primary root emerges from the seed and grows downwards. Then, the plant’s primary leaf emerges and grows upwards. It is protected by a cylindrical, hollow structure known as coleoptile. Once the seedling grows above soil surface, the growth of coleoptile stops and it is pierced by the primary leaf.

Of course, all the seeds that happen to land in soil are not equally lucky to get the proper environment to germinate. Many seeds tend to dry up and cannot develop into a plant. Only those seeds that get sufficient amounts of water, oxygen, and the right temperature along with soil germinate into plants.

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The life cycle of a flowering plant starts with a seed. The seed germinates to produce a sapling, which matures into a plant. This plant then reproduces to form new&hellip

Read on to know more about the steps of photosynthesis, one of nature's most fascinating occurrence.


Land plants evolved from a group of green algae, perhaps as early as 850 mya, [8] but algae-like plants might have evolved as early as 1 billion years ago. [7] The closest living relatives of land plants are the charophytes, specifically Charales assuming that the habit of the Charales has changed little since the divergence of lineages, this means that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, [10] perhaps at the edge of seasonally desiccating pools. [11] However, some recent evidence suggests that land plants might have originated from unicellular terrestrial charophytes similar to extant Klebsormidiophyceae. [12] The alga would have had a haplontic life cycle. It would only very briefly have had paired chromosomes (the diploid condition) when the egg and sperm first fused to form a zygote that would have immediately divided by meiosis to produce cells with half the number of unpaired chromosomes (the haploid condition). Co-operative interactions with fungi may have helped early plants adapt to the stresses of the terrestrial realm. [13]

Plants were not the first photosynthesisers on land. Weathering rates suggest that organisms capable of photosynthesis were already living on the land 1,200 million years ago , [11] and microbial fossils have been found in freshwater lake deposits from 1,000 million years ago , [14] but the carbon isotope record suggests that they were too scarce to impact the atmospheric composition until around 850 million years ago . [8] These organisms, although phylogenetically diverse, [15] were probably small and simple, forming little more than an algal scum. [11]

Evidence of the earliest land plants occurs much later at about 470Ma, in lower middle Ordovician rocks from Saudi Arabia [16] and Gondwana [17] in the form of spores with decay-resistant walls. These spores, known as cryptospores, were produced either singly (monads), in pairs (dyads) or groups of four (tetrads), and their microstructure resembles that of modern liverwort spores, suggesting they share an equivalent grade of organisation. [18] Their walls contain sporopollenin – further evidence of an embryophytic affinity. [19] It could be that atmospheric 'poisoning' prevented eukaryotes from colonising the land prior to this, [20] or it could simply have taken a great time for the necessary complexity to evolve. [21]

Trilete spores similar to those of vascular plants appear soon afterwards, in Upper Ordovician rocks about 455 million years ago. [22] [23] Depending exactly when the tetrad splits, each of the four spores may bear a "trilete mark", a Y-shape, reflecting the points at which each cell squashed up against its neighbours. [24] However, this requires that the spore walls be sturdy and resistant at an early stage. This resistance is closely associated with having a desiccation-resistant outer wall—a trait only of use when spores must survive out of water. Indeed, even those embryophytes that have returned to the water lack a resistant wall, thus don't bear trilete marks. [24] A close examination of algal spores shows that none have trilete spores, either because their walls are not resistant enough, or, in those rare cases where they are, because the spores disperse before they are compressed enough to develop the mark or do not fit into a tetrahedral tetrad. [24]

The earliest megafossils of land plants were thalloid organisms, which dwelt in fluvial wetlands and are found to have covered most of an early Silurian flood plain. They could only survive when the land was waterlogged. [25] There were also microbial mats. [26]

Once plants had reached the land, there were two approaches to dealing with desiccation. Modern bryophytes either avoid it or give in to it, restricting their ranges to moist settings or drying out and putting their metabolism "on hold" until more water arrives, as in the liverwort genus Targionia. Tracheophytes resist desiccation by controlling the rate of water loss. They all bear a waterproof outer cuticle layer wherever they are exposed to air (as do some bryophytes), to reduce water loss, but since a total covering would cut them off from CO
2 in the atmosphere tracheophytes use variable openings, the stomata, to regulate the rate of gas exchange. Tracheophytes also developed vascular tissue to aid in the movement of water within the organisms (see below), and moved away from a gametophyte dominated life cycle (see below). Vascular tissue ultimately also facilitated upright growth without the support of water and paved the way for the evolution of larger plants on land.

A snowball earth, from around 850-630 mya, is believed to have been caused by early photosynthetic organisms, which reduced the concentration of carbon dioxide and increased the amount of oxygen in the atmosphere. [27] The establishment of a land-based flora increased the rate of accumulation of oxygen in the atmosphere, as the land plants produced oxygen as a waste product. When this concentration rose above 13%, around 2.45 billion years ago, [28] wildfires became possible, evident from charcoal in the fossil record. [29] Apart from a controversial gap in the Late Devonian, charcoal is present ever since.

Charcoalification is an important taphonomic mode. Wildfire or burial in hot volcanic ash drives off the volatile compounds, leaving only a residue of pure carbon. This is not a viable food source for fungi, herbivores or detritovores, so it is prone to preservation. It is also robust and can withstand pressure, displaying exquisite, sometimes sub-cellular, detail in remains.

All multicellular plants have a life cycle comprising two generations or phases. The gametophyte phase has a single set of chromosomes (denoted 1n) and produces gametes (sperm and eggs). The sporophyte phase has paired chromosomes (denoted 2n) and produces spores. The gametophyte and sporophyte phases may be homomorphic, appearing identical in some algae, such as Ulva lactuca, but are very different in all modern land plants, a condition known as heteromorphy.

The pattern in plant evolution has been a shift from homomorphy to heteromorphy. The algal ancestors of land plants were almost certainly haplobiontic, being haploid for all their life cycles, with a unicellular zygote providing the 2N stage. All land plants (i.e. embryophytes) are diplobiontic – that is, both the haploid and diploid stages are multicellular. [6] Two trends are apparent: bryophytes (liverworts, mosses and hornworts) have developed the gametophyte as the dominant phase of the life cycle, with the sporophyte becoming almost entirely dependent on it vascular plants have developed the sporophyte as the dominant phase, with the gametophytes being particularly reduced in the seed plants.

It has been proposed as the basis for the emergence of the diploid phase of the life cycle as the dominant phase that diploidy allows masking of the expression of deleterious mutations through genetic complementation. [30] [31] Thus if one of the parental genomes in the diploid cells contains mutations leading to defects in one or more gene products, these deficiencies could be compensated for by the other parental genome (which nevertheless may have its own defects in other genes). As the diploid phase was becoming predominant, the masking effect likely allowed genome size, and hence information content, to increase without the constraint of having to improve accuracy of replication. The opportunity to increase information content at low cost is advantageous because it permits new adaptations to be encoded. This view has been challenged, with evidence showing that selection is no more effective in the haploid than in the diploid phases of the lifecycle of mosses and angiosperms. [32]

There are two competing theories to explain the appearance of a diplobiontic lifecycle.

The interpolation theory (also known as the antithetic or intercalary theory) [33] holds that the interpolation of a multicellular sporophyte phase between two successive gametophyte generations was an innovation caused by preceding meiosis in a freshly germinated zygote with one or more rounds of mitotic division, thereby producing some diploid multicellular tissue before finally meiosis produced spores. This theory implies that the first sporophytes bore a very different and simpler morphology to the gametophyte they depended on. [33] This seems to fit well with what is known of the bryophytes, in which a vegetative thalloid gametophyte nurtures a simple sporophyte, which consists of little more than an unbranched sporangium on a stalk. Increasing complexity of the ancestrally simple sporophyte, including the eventual acquisition of photosynthetic cells, would free it from its dependence on a gametophyte, as seen in some hornworts (Anthoceros), and eventually result in the sporophyte developing organs and vascular tissue, and becoming the dominant phase, as in the tracheophytes (vascular plants). [6] This theory may be supported by observations that smaller Cooksonia individuals must have been supported by a gametophyte generation. The observed appearance of larger axial sizes, with room for photosynthetic tissue and thus self-sustainability, provides a possible route for the development of a self-sufficient sporophyte phase. [33]

The alternative hypothesis, called the transformation theory (or homologous theory), posits that the sporophyte might have appeared suddenly by delaying the occurrence of meiosis until a fully developed multicellular sporophyte had formed. Since the same genetic material would be employed by both the haploid and diploid phases, they would look the same. This explains the behaviour of some algae, such as Ulva lactuca, which produce alternating phases of identical sporophytes and gametophytes. Subsequent adaption to the desiccating land environment, which makes sexual reproduction difficult, might have resulted in the simplification of the sexually active gametophyte, and elaboration of the sporophyte phase to better disperse the waterproof spores. [6] The tissue of sporophytes and gametophytes of vascular plants such as Rhynia preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis. [33] [34] [35] By contrast, modern vascular plants, with the exception of Psilotum, have heteromorphic sporophytes and gametophytes in which the gametophytes rarely have any vascular tissue. [36]

Arbuscular mycorrhizal symbiosis Edit

There is no evidence that early land plants of the Silurian and early Devonian had roots, although fossil evidence of rhizoids occurs for several species, such as Horneophyton. The earliest land plants did not have vascular systems for transport of water and nutrients either. Aglaophyton, a rootless vascular plant known from Devonian fossils in the Rhynie chert [37] was the first land plant discovered to have had a symbiotic relationship with fungi [38] which formed arbuscular mycorrhizas, literally "tree-like fungal roots", in a well-defined cylinder of cells (ring in cross section) in the cortex of its stems. The fungi fed on the plant's sugars, in exchange for nutrients generated or extracted from the soil (especially phosphate), to which the plant would otherwise have had no access. Like other rootless land plants of the Silurian and early Devonian Aglaophyton may have relied on arbuscular mycorrhizal fungi for acquisition of water and nutrients from the soil.

The fungi were of the phylum Glomeromycota, [39] a group that probably first appeared 1 billion years ago and still forms arbuscular mycorrhizal associations today with all major land plant groups from bryophytes to pteridophytes, gymnosperms and angiosperms and with more than 80% of vascular plants. [40]

Evidence from DNA sequence analysis indicates that the arbuscular mycorrhizal mutualism arose in the common ancestor of these land plant groups during their transition to land [41] and it may even have been the critical step that enabled them to colonise the land. [42] Appearing as they did before these plants had evolved roots, mycorrhizal fungi would have assisted plants in the acquisition of water and mineral nutrients such as phosphorus, in exchange for organic compounds which they could not synthesize themselves. [40] Such fungi increase the productivity even of simple plants such as liverworts. [43] [44]

Cuticle, stomata and intercellular spaces Edit

To photosynthesise, plants must absorb CO
2 from the atmosphere. However, making the tissues available for CO
2 to enter allows water to evaporate, so this comes at a price. [45] Water is lost much faster than CO
2 is absorbed, so plants need to replace it. Early land plants transported water apoplastically, within the porous walls of their cells. Later, they evolved three anatomical features that provided the ability to control the inevitable water loss that accompanied CO
2 acquisition. First, a waterproof outer covering or cuticle evolved that reduced water loss. Secondly, variable apertures, the stomata that could open and close to regulate the amount of water lost by evaporation during CO
2 uptake and thirdly intercellular space between photosynthetic parenchyma cells that allowed improved internal distribution of the CO
2 to the chloroplasts. This three-part system provided improved homoiohydry, the regulation of water content of the tissues, providing a particular advantage when water supply is not constant. [46] The high CO
2 concentrations of the Silurian and early Devonian, when plants were first colonising land, meant that they used water relatively efficiently. As CO
2 was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant water acquisition and transport mechanisms evolved. [45] Plants growing upwards into the air needed a system for transporting water from the soil to all the different parts of the above-soil plant, especially to photosynthesising parts. By the end of the Carboniferous, when CO
2 concentrations had been reduced to something approaching that of today, around 17 times more water was lost per unit of CO
2 uptake. [45] However, even in the "easy" early days, water was always at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation. [46]

Water can be wicked by capillary action along a fabric with small spaces. In narrow columns of water, such as those within the plant cell walls or in tracheids, when molecules evaporate from one end, they pull the molecules behind them along the channels. Therefore, evaporation alone provides the driving force for water transport in plants. [45] However, without specialized transport vessels, this cohesion-tension mechanism can cause negative pressures sufficient to collapse water conducting cells, limiting the transport water to no more than a few cm, and therefore limiting the size of the earliest plants. [45]

Xylem Edit

To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system. As plants grew upwards, specialised water transport vascular tissues evolved, first in the form of simple hydroids of the type found in the setae of moss sporophytes. These simple elongated cells were dead and water-filled at maturity, providing a channel for water transport, but their thin, unreinforced walls would collapse under modest water tension, limiting the plant height. Xylem tracheids, wider cells with lignin-reinforced cell walls that were more resistant to collapse under the tension caused by water stress, occur in more than one plant group by mid-Silurian, and may have a single evolutionary origin, possibly within the hornworts, [47] uniting all tracheophytes. Alternatively, they may have evolved more than once. [45] Much later, in the Cretaceous, tracheids were followed by vessels in flowering plants. [45] As water transport mechanisms and waterproof cuticles evolved, plants could survive without being continually covered by a film of water. This transition from poikilohydry to homoiohydry opened up new potential for colonisation. [45] [46]

The early Devonian pretracheophytes Aglaophyton and Horneophyton have unreinforced water transport tubes with wall structures very similar to moss hydroids, but they grew alongside several species of tracheophytes, such as Rhynia gwynne-vaughanii that had xylem tracheids that were well reinforced by bands of lignin. The earliest macrofossils known to have xylem tracheids are small, mid-Silurian plants of the genus Cooksonia. [48] However, thickened bands on the walls of isolated tube fragments are apparent from the early Silurian onwards. [49]

Plants continued to innovate ways of reducing the resistance to flow within their cells, progressively increasing the efficiency of their water transport and to increase the resistance of the tracheids to collapse under tension. [50] [51] During the early Devonian, maximum tracheid diameter increased with time, but may have plateaued in the zosterophylls by mid-Devonian. [50] Overall transport rate also depends on the overall cross-sectional area of the xylem bundle itself, and some mid-Devonian plants, such as the Trimerophytes, had much larger steles than their early ancestors. [50] While wider tracheids provided higher rates of water transport, they increased the risk of cavitation, the formation of air bubbles resulting from the breakage of the water column under tension. [45] Small pits in tracheid walls allow water to by-pass a defective tracheid while preventing air bubbles from passing through [45] but at the cost of restricted flow rates. By the Carboniferous, Gymnosperms had developed bordered pits, [52] [53] valve-like structures that allow high-conductivity pits to seal when one side of a tracheid is depressurized.

Tracheids have perforated end walls, which impose a great deal of resistance on water flow, [50] but may have had the advantage of isolating air embolisms caused by cavitation or freezing. Vessels first evolved during the dry, low CO
2 periods of the Late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in gnetophytes and angiosperms. [45] Vessel members are open tubes with no end walls, and are arranged end to end to operate as if they were one continuous vessel. [50] Vessels allowed the same cross-sectional area of wood to transport much more water than tracheids. [45] This allowed plants to fill more of their stems with structural fibres and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on. [45] Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation. [45] Once plants had evolved this level of control over water evaporation and water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size [46] [45] but as a result of their increased independence from their surroundings, most vascular plants lost their ability to survive desiccation - a costly trait to lose. [45] In early land plants, support was mainly provided by turgor pressure, particularly of the outer layer of cells known as the sterome tracheids, and not by the xylem, which was too small, too weak and in too central a position to provide much structural support. [45] Plants with secondary xylem that had appeared by mid-Devonian, such as the Trimerophytes and Progymnosperms had much larger vascular cross sections producing strong woody tissue.

Endodermis Edit

An endodermis may have evolved in the earliest plant roots during the Devonian, but the first fossil evidence for such a structure is Carboniferous. [45] The endodermis in the roots surrounds the water transport tissue and regulates ion exchange between the groundwater and the tissues and prevents unwanted pathogens etc. from entering the water transport system. The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.

Leaves Edit

Leaves are the primary photosynthetic organs of a modern plant. The origin of leaves was almost certainly triggered by falling concentrations of atmospheric CO
2 during the Devonian period, increasing the efficiency with which carbon dioxide could be captured for photosynthesis. [54] [55]

Leaves certainly evolved more than once. Based on their structure, they are classified into two types: microphylls, which lack complex venation and may have originated as spiny outgrowths known as enations, and megaphylls, which are large and have complex venation that may have arisen from the modification of groups of branches. It has been proposed that these structures arose independently. [56] Megaphylls, according to Walter Zimmerman's telome theory, [57] have evolved from plants that showed a three-dimensional branching architecture, through three transformations—overtopping, which led to the lateral position typical of leaves, planation, which involved formation of a planar architecture, webbing or fusion, which united the planar branches, thus leading to the formation of a proper leaf lamina. All three steps happened multiple times in the evolution of today's leaves. [58]

It is widely believed that the telome theory is well supported by fossil evidence. However, Wolfgang Hagemann questioned it for morphological and ecological reasons and proposed an alternative theory. [59] [60] Whereas according to the telome theory the most primitive land plants have a three-dimensional branching system of radially symmetrical axes (telomes), according to Hagemann's alternative the opposite is proposed: the most primitive land plants that gave rise to vascular plants were flat, thalloid, leaf-like, without axes, somewhat like a liverwort or fern prothallus. Axes such as stems and roots evolved later as new organs. Rolf Sattler proposed an overarching process-oriented view that leaves some limited room for both the telome theory and Hagemann's alternative and in addition takes into consideration the whole continuum between dorsiventral (flat) and radial (cylindrical) structures that can be found in fossil and living land plants. [61] [62] This view is supported by research in molecular genetics. Thus, James (2009) [63] concluded that "it is now widely accepted that. radiality [characteristic of axes such as stems] and dorsiventrality [characteristic of leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!"

Before the evolution of leaves, plants had the photosynthetic apparatus on the stems. Today's megaphyll leaves probably became commonplace some 360mya, about 40my after the simple leafless plants had colonized the land in the Early Devonian. This spread has been linked to the fall in the atmospheric carbon dioxide concentrations in the Late Paleozoic era associated with a rise in density of stomata on leaf surface. [54] This would have resulted in greater transpiration rates and gas exchange, but especially at high CO
2 concentrations, large leaves with fewer stomata would have heated to lethal temperatures in full sunlight. Increasing the stomatal density allowed for a better-cooled leaf, thus making its spread feasible, but increased CO2 uptake at the expense of decreased water use efficiency. [55] [64]

The rhyniophytes of the Rhynie chert consisted of nothing more than slender, unornamented axes. The early to middle Devonian trimerophytes may be considered leafy. This group of vascular plants are recognisable by their masses of terminal sporangia, which adorn the ends of axes which may bifurcate or trifurcate. [6] Some organisms, such as Psilophyton, bore enations. These are small, spiny outgrowths of the stem, lacking their own vascular supply.

The zosterophylls were already important in the late Silurian, much earlier than any rhyniophytes of comparable complexity. [65] This group, recognisable by their kidney-shaped sporangia which grew on short lateral branches close to the main axes, sometimes branched in a distinctive H-shape. [6] Many zosterophylls bore pronounced spines on their axes [ citation needed ] but none of these had a vascular trace. The first evidence of vascularised enations occurs in a fossil clubmoss known as Baragwanathia that had already appeared in the fossil record in the Late Silurian. [66] In this organism, these leaf traces continue into the leaf to form their mid-vein. [67] One theory, the "enation theory", holds that the microphyllous leaves of clubmosses developed by outgrowths of the protostele connecting with existing enations [6] The leaves of the Rhynie genus Asteroxylon, which was preserved in the Rhynie chert almost 20 Million years later than Baragwanathia had a primitive vascular supply – in the form of leaf traces departing from the central protostele towards each individual "leaf". [68] Asteroxylon and Baragwanathia are widely regarded as primitive lycopods, [6] a group still extant today, represented by the quillworts, the spikemosses and the club mosses. Lycopods bear distinctive microphylls, defined as leaves with a single vascular trace. Microphylls could grow to some size, those of Lepidodendrales reaching over a meter in length, but almost all just bear the one vascular bundle. An exception is the rare branching in some Selaginella species.

The more familiar leaves, megaphylls, are thought to have originated four times independently, in the ferns, horsetails, progymnosperms and seed plants. [69] They appear to have originated by modifying dichotomising branches, which first overlapped (or "overtopped") one another, became flattened or planated and eventually developed "webbing" and evolved gradually into more leaf-like structures. [67] Megaphylls, by Zimmerman's telome theory, are composed of a group of webbed branches [67] and hence the "leaf gap" left where the leaf's vascular bundle leaves that of the main branch resembles two axes splitting. [67] In each of the four groups to evolve megaphylls, their leaves first evolved during the Late Devonian to Early Carboniferous, diversifying rapidly until the designs settled down in the mid Carboniferous. [69]

The cessation of further diversification can be attributed to developmental constraints, [69] but why did it take so long for leaves to evolve in the first place? Plants had been on the land for at least 50 million years before megaphylls became significant. However, small, rare mesophylls are known from the early Devonian genus Eophyllophyton – so development could not have been a barrier to their appearance. [70] The best explanation so far incorporates observations that atmospheric CO
2 was declining rapidly during this time – falling by around 90% during the Devonian. [71] This required an increase in stomatal density by 100 times to maintain rates of photosynthesis. When stomata open to allow water to evaporate from leaves it has a cooling effect, resulting from the loss of latent heat of evaporation. It appears that the low stomatal density in the early Devonian meant that evaporation and evaporative cooling were limited, and that leaves would have overheated if they grew to any size. The stomatal density could not increase, as the primitive steles and limited root systems would not be able to supply water quickly enough to match the rate of transpiration. [55] Clearly, leaves are not always beneficial, as illustrated by the frequent occurrence of secondary loss of leaves, famously exemplified by cacti and the "whisk fern" Psilotum.

Secondary evolution can also disguise the true evolutionary origin of some leaves. Some genera of ferns display complex leaves which are attached to the pseudostele by an outgrowth of the vascular bundle, leaving no leaf gap. [67] Further, horsetail (Equisetum) leaves bear only a single vein, and appear to be microphyllous however, both the fossil record and molecular evidence indicate that their forebears bore leaves with complex venation, and the current state is a result of secondary simplification. [72]

Deciduous trees deal with another disadvantage to having leaves. The popular belief that plants shed their leaves when the days get too short is misguided evergreens prospered in the Arctic circle during the most recent greenhouse earth. [73] The generally accepted reason for shedding leaves during winter is to cope with the weather – the force of wind and weight of snow are much more comfortably weathered without leaves to increase surface area. Seasonal leaf loss has evolved independently several times and is exhibited in the ginkgoales, some pinophyta and certain angiosperms. [74] Leaf loss may also have arisen as a response to pressure from insects it may have been less costly to lose leaves entirely during the winter or dry season than to continue investing resources in their repair. [75]

Factors influencing leaf architectures Edit

Various physical and physiological factors such as light intensity, humidity, temperature, wind speeds etc. have influenced evolution of leaf shape and size. High trees rarely have large leaves, because they are damaged by high winds. Similarly, trees that grow in temperate or taiga regions have pointed leaves [76] presumably to prevent nucleation of ice onto the leaf surface and reduce water loss due to transpiration. Herbivory, by mammals and insects, has been a driving force in leaf evolution. An example is that plants of the New Zealand genus Aciphylla have spines on their laminas, which probably functioned to discourage the extinct Moas from feeding on them. Other members of Aciphylla, which did not co-exist with the moas, do not have these spines. [77]

At the genetic level, developmental studies have shown that repression of KNOX genes is required for initiation of the leaf primordium. This is brought about by ARP genes, which encode transcription factors. Repression of KNOX genes in leaf primordia seems to be quite conserved, while expression of KNOX genes in leaves produces complex leaves. The ARP function appears to have arisen early in vascular plant evolution, because members of the primitive group Lycophytes also have a functionally similar gene. [78] Other players that have a conserved role in defining leaf primordia are the phytohormones auxin, gibberelin and cytokinin.

The arrangement of leaves or phyllotaxy on the plant body can maximally harvest light and might be expected to be genetically robust. However, in maize, a mutation in only one gene called ABPHYL (ABnormal PHYLlotaxy) is enough to change the phyllotaxy of the leaves, implying that mutational adjustment of a single locus on the genome is enough to generate diversity. [79]

Once the leaf primordial cells are established from the SAM cells, the new axes for leaf growth are defined, among them being the abaxial-adaxial (lower-upper surface) axes. The genes involved in defining this, and the other axes seem to be more or less conserved among higher plants. Proteins of the HD-ZIPIII family have been implicated in defining the adaxial identity. These proteins deviate some cells in the leaf primordium from the default abaxial state, and make them adaxial. In early plants with leaves, the leaves probably just had one type of surface — the abaxial one, the underside of today's leaves. The definition of the adaxial identity occurred some 200 million years after the abaxial identity was established. [80]

How the wide variety of observed plant leaf morphology is generated is a subject of intense research. Some common themes have emerged. One of the most significant is the involvement of KNOX genes in generating compound leaves, as in the tomato (see above). But, this is not universal. For example, the pea uses a different mechanism for doing the same thing. [81] [82] Mutations in genes affecting leaf curvature can also change leaf form, by changing the leaf from flat, to a crinkly shape, [83] like the shape of cabbage leaves. There also exist different morphogen gradients in a developing leaf which define the leaf's axis and may also affect the leaf form. Another class of regulators of leaf development are the microRNAs. [84] [85]

Roots Edit

The roots (bottom image) of Lepidodendrales (Stigmaria) are thought to be developmentally equivalent to the stems (top), as the similar appearance of "leaf scars" and "root scars" on these specimens from different species demonstrates.

Roots are important to plants for two main reasons: Firstly, they provide anchorage to the substrate more importantly, they provide a source of water and nutrients from the soil. Roots allowed plants to grow taller and faster.

The evolution of roots had consequences on a global scale. By disturbing the soil and promoting its acidification (by taking up nutrients such as nitrate and phosphate [86] ), they enabled it to weather more deeply, injecting carbon compounds deeper into soils [87] with huge implications for climate. [88] These effects may have been so profound they led to a mass extinction. [89]

While there are traces of root-like impressions in fossil soils in the Late Silurian, [90] body fossils show the earliest plants to be devoid of roots. Many had prostrate branches that sprawled along the ground, with upright axes or thalli dotted here and there, and some even had non-photosynthetic subterranean branches which lacked stomata. The distinction between root and specialised branch is developmental. [ clarification needed ] differing in their branching pattern and in possession of a root cap. [11] So while Siluro-Devonian plants such as Rhynia and Horneophyton possessed the physiological equivalent of roots, [91] [92] roots – defined as organs differentiated from stems – did not arrive until later. [11] Unfortunately, roots are rarely preserved in the fossil record, and our understanding of their evolutionary origin is sparse. [11]

Rhizoids – small structures performing the same role as roots, usually a cell in diameter – probably evolved very early, perhaps even before plants colonised the land they are recognised in the Characeae, an algal sister group to land plants. [11] That said, rhizoids probably evolved more than once the rhizines of lichens, for example, perform a similar role. Even some animals (Lamellibrachia) have root-like structures. [11] Rhizoids are clearly visible in the Rhynie chert fossils, and were present in most of the earliest vascular plants, and on this basis seem to have presaged true plant roots. [93]

More advanced structures are common in the Rhynie chert, and many other fossils of comparable early Devonian age bear structures that look like, and acted like, roots. [11] The rhyniophytes bore fine rhizoids, and the trimerophytes and herbaceous lycopods of the chert bore root-like structure penetrating a few centimetres into the soil. [94] However, none of these fossils display all the features borne by modern roots, [11] with the exception of Asteroxylon, which has recently been recognized as bearing roots that evolved independently from those of extant vascular plants. [95] Roots and root-like structures became increasingly common and deeper penetrating during the Devonian, with lycopod trees forming roots around 20 cm long during the Eifelian and Givetian. These were joined by progymnosperms, which rooted up to about a metre deep, during the ensuing Frasnian stage. [94] True gymnosperms and zygopterid ferns also formed shallow rooting systems during the Famennian. [94]

The rhizophores of the lycopods provide a slightly different approach to rooting. They were equivalent to stems, with organs equivalent to leaves performing the role of rootlets. [11] A similar construction is observed in the extant lycopod Isoetes, and this appears to be evidence that roots evolved independently at least twice, in the lycophytes and other plants, [11] a proposition supported by studies showing that roots are initiated and their growth promoted by different mechanisms in lycophytes and euphyllophytes. [96]

A vascular system is indispensable to rooted plants, as non-photosynthesising roots need a supply of sugars, and a vascular system is required to transport water and nutrients from the roots to the rest of the plant. [10] Rooted plants [ which? ] are little more advanced than their Silurian forebears, without a dedicated root system however, the flat-lying axes can be clearly seen to have growths similar to the rhizoids of bryophytes today. [97]

By the Middle to Late Devonian, most groups of plants had independently developed a rooting system of some nature. [97] As roots became larger, they could support larger trees, and the soil was weathered to a greater depth. [89] This deeper weathering had effects not only on the aforementioned drawdown of CO
2 , but also opened up new habitats for colonisation by fungi and animals. [94]

Roots today have developed to the physical limits. They penetrate as much as 60 metres of soil to tap the water table. [98] The narrowest roots are a mere 40 μm in diameter, and could not physically transport water if they were any narrower. [11] The earliest fossil roots recovered, by contrast, narrowed from 3 mm to under 700 μm in diameter of course, taphonomy is the ultimate control of what thickness can be seen. [11]

Watch the video: The Roots - Why Whats Goin On (August 2022).