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I have been doing some gardening recently and I suddenly realised that all plants have superficially identical non-woody roots of the same size from gigantic trees to small fruiting plants and vegetables. They are all white, no more than 1 mm across, long and flexible with no distinguishing marks. Only when the root start to become woody do they become different in appearance.

Leaves are very diverse even when they have just started to grow, i.e. before they have reached their full size. I can recognise many different plants from their smallest leaves alone but I wouldn't be able to differentiate plants by looking at their roots. So why aren't roots anywhere near as diverse as leaves?

a potato from my garden

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    $\begingroup$ If you consider rhizomes, corms, bulbs, tubers, nitrogen-fixing nodules, etc. as part of the root system, there's more variety than you give credit for. But an interesting question none-the-less. +1. $\endgroup$ – anongoodnurse Dec 27 '15 at 4:42
  • $\begingroup$ @anongoodnurse A potato, which is a tuber, will still put out roots that look like any other plants'. But if you consider that almost every plant can be visually identified by its leaves alone, why not roots? $\endgroup$ – CJ Dennis Dec 27 '15 at 4:56
  • $\begingroup$ I was agreeing with you; the roots are very similar. But I wouldn't go so far as to say that every plant can be identified by its leaves alone, if you want me to disagree with you. :) $\endgroup$ – anongoodnurse Dec 27 '15 at 5:02
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    $\begingroup$ I think the perceived diversity is biased by the relative obscurity of roots and familiarity of leaves as well as the scale at which a human observes these organs with the naked eye. I suspect functional diversity of roots is greater than leaves. One reason for this is that roots compete for and acquire a more diverse set of resources and interact with a larger diversity of organisms than do leaves. In short, if you could observe the metabolome and microbiome I would expect the opposite to be true. $\endgroup$ – David LeBauer Dec 28 '15 at 5:28
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    $\begingroup$ @anongoodnurse Underground parts does not necessarily means root. Corms and rhizomes are stem-modification. Tuber is quite informal term and tuber commonly indicate stem-tuber. (However some storage roots are also called root-tubers or tuberous roots). Bulbs of lilies and amaryllids are usually a structure made up of stem, leaves as well as roots. Basic types of plant-organs, like root, stems and leaves are well-demarcated from their developmental-nature and anatomy. $\endgroup$ – Always Confused Sep 10 '16 at 3:59
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To answer this type of question, we should start by defining 2 concepts: the functions of the root and the external condition that brought to a specific evolution. Evolution is a necessary process to all organisms, which evolve to optimize a specific function and to better adapt to the external conditions.

There are 3 main and common functions between all land plant's roots: fix plants to the soil; absorb water and minerals; establish specific relationships with other organisms, principally fungi and bacteria. The external conditions of roots (soil) are quite stable, even though, the different "physical" composition of soil, that is the quantity of sand, clay and rocks, change the physical appearance of size and proportion between the main and the lateral roots.Therefore it is plausible to say that it is not necessary to have a great variety in root's structure; since all roots must carry out same functions, all plant's root underwent a similar evolution.

For the leaves it is a bit different. Leaves must also carry out specific and common functions, as: exchange gas (CO2 and O2); contain photoreceptors to "absorb" light for photosynthesis. However, the external conditions of leaves are shortly and continuously changing and highly different (consider the vary regions of the world, as desert and rainforest) with reference to roots. Therefore each plants underwent evolution depending on these types of external conditions, so as to, of course, optimize their functions but at the same time protect themselves as much as possible. Consider, for example, the intensity of sun's radiation, which, if too strong, can destroy cellular structures, the quantity of CO2 in the air, or the high biodiversity of insects, which can carry diseases, ecc...

Interesting book about evolution is: J.C. Harmon, S. Freeman, "Evolutionary analysis", where it is given an idea of the type of evolution, different species, could undergo. (hope my english is not too bad)

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I originally listed two reasons for the relative lack of diversity among roots - the substrate and roots' primary function, transporting water and nutrients. After giving it more thought, I've expanded the list to four reasons.

1. SUBSTRATE - Burrowing animals are far less diverse than animals that live aboveground; note the remarkable similarity between moles, golden moles and marsupial moles - all unrelated. The obvious reason is that they're adapted to living in soil. Roots are similarly constrained by their environment. It's hard to imagine a giant palm leaf growing underground.

2. UNIFORM ENVIRONMENT - Leaves experience light and dark, daily temperature fluctuations and even greater seasonal fluctuations. They may be pummeled by high winds, hail and fire. These variables may influence leaf evolution in many different ways. In contrast, roots grow in an environment that is always dark, relatively protected from wind, hail and fire and relatively stable temperature wise.

3. FUNCTION - Aside from anchoring plants, roots' primary function is transporting water and nutrients. The optimum shape for this is round, similar to the veins in our bodies and the plumbing in people's homes. Rectangular roots or roots shaped like maple leaves make no sense.

4. PREDATORS - A wide variety of animals feed on leaves, which may evolve defenses against predation. For example, a cactus' spines are modified leaves that are widely believed to serve as defenses against animals in search of a juicy cactus. In contrast, relatively few animals feed on roots. There are a few species that dig for roots, and some burrowing animals feed on roots, but virtually no wild primates or ungulates feed on roots.

In summary, leaves grow in an environment where they are less constrained and less protected by the surrounding substrate and are exposed to a far greater variety of environmental variables and predators.

As far as color goes, I don't see much difference; most roots are whitish or brownish, while most leaves are green. Of course, leaves of some species do change colors in the fall, when temperatures begin to decrease. But, once again, roots are relatively insulated from the cold, especially if the ground is covered with a blanket of snow.

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  • $\begingroup$ As an example, pine trees have round needle-like leaves whereas grass has flat leaves. Why not flat roots versus round roots? Obviously when it starts to grow it must be small. A tree isn't going to grow a 10 cm wide root. It will grow a 1 mm root and gradually thicken the older parts as the length increases. Why are long (minimally branching) round roots seemingly optimal in every environment? $\endgroup$ – CJ Dennis Dec 27 '15 at 3:47
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    $\begingroup$ While of value, this is just an opinion posted as an answer. References make an answer authoritative. I am doubtful that you can find good references explaining the reason as you did. Leaves are incredibly diverse. Roots, only somewhat so. $\endgroup$ – anongoodnurse Dec 27 '15 at 4:40
  • $\begingroup$ Huh? I think everyone here agrees that are leaves are incredibly diverse, including the OP and myself. I also think there's a little room for common sense here. If you think really thick roots would have no problem penetrating soil and rocks, maybe you could explain why. $\endgroup$ – David Blomstrom Dec 27 '15 at 6:46
  • $\begingroup$ As for flat vs round, leaves' primary function is capturing sunlight. Thus, they have evolved to maximize surface area (i.e. they're generally flat). Various evolutionary pressures have modified some leaves into needles, but that's an aside. Roots function largely in transporting water (and nutrients), similar to blood vessels. Round appears to be the optimum shape for this task. $\endgroup$ – David Blomstrom Dec 27 '15 at 6:49
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    $\begingroup$ Please could you add some supporting scientific material that reinforces your answer and allows further reading. $\endgroup$ – rg255 Mar 8 '16 at 7:26

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