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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. $\endgroup$ Dec 27, 2015 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, 2015 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. :) $\endgroup$ Dec 27, 2015 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. $\endgroup$ Dec 28, 2015 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. $\endgroup$
    – user25568
    Sep 10, 2016 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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