What Was The Reason Of Development Of Leaves And Roots In Early Vascular Plants?

Early vascular land plants lack leaves or roots. They were small in size, with an erect aerial portion and an underground absorptive zone called rhizome. With the passage of time there was an increase in their size i.e., the aerial portion became more massive and complicated. Such alternations in the basic architecture of the plant body required much more efficient and quick system of photosynthesis for synthesizing carbohydrates rapidly and more stable and strong anchorage and absorptive system. In response to such requirements early vascular plants develop leaves and roots.
Scale like outgrowths:
Early land plants such as Psilophyton, Cooksonia and Rhynia had dichotomously branched (each branch gives rise to two branches i.e., forked branching system) smooth aerial parts with equally strong underground rhizome. They lived about 400 to 390 million years ago.Rhynia also had the same structural layout i.e., naked stem without leaves. Such plants started to form leaves as small scale like outgrowths. These outgrowths were not supplies with vascular bundles; therefore they were not regarded as true leaves.

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