Evolution Of Leaves

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During the evolution of plant leaves have originated at least two separate occasions. The legacy of these historical evolutionary events is represented in the floras current by microphylls of lycophytes ( club mosses , Selaginella and Isoetes ) and the eufilofitas megafilos ( ferns , gymnosperms and angiosperms ). It is considered that the microphylls, with its characteristic vasculature and its absence of branching, have evolved from thorny excrescences and megafilos predate the fossil record of terrestrial plants. Of greater importance, however, was the source of vascular plants in megafilos through the modification of the development of lateral branches since the great majority of the 250,000 flowering plant species existing as well as most gymnosperms, use a flat sheet megafilo a network of veins to capture solar energy and photosynthetic carbon assimilation. The megafilos altered, in fact, the evolutionary path of plants and terrestrial animals, biogeochemical cycles of nutrients, water and carbon dioxide and energy exchange between the land surface and atmosphere. 45 A measure of success in terrestrial environments is ...
... the ability of leaves to withstand extreme weather conditions between the tropics and the tundra , while at the same time facilitate the net fixation of about 207 billion tons of CO 2 (56.4 × 10 15 g C) year -1 globally. This primary production provides energy for almost all forms of terrestrial life on Earth , especially tetrapods and insects , and link many ecosystems and biogeochemical processes.

Obviously, the leaves are evolutionary success of land plants. However, the advent of large megafilos took place around 40-50 million years after the origin of vascular plants, suggesting that they were far from being an inevitable consequence of evolution. The earliest ancestral vascular plants, dating from the late Silurian , 410 million years ago, consisted of a simple or branched stems with sporangia but no leaves. 48 Surprisingly, plants lacking leaves continued for the next 40-50 million years, when finally megafilos widespread towards the end of the period Devonian , 360 million years ago. 49 50 51 52 This late appearance of the leaves, a seemingly simple structural modification, it is surprising for at least three reasons. First, the paleontological evidence shows that the structural framework required for assembly of a single sheet evolutionary (meristems, vascularization, cuticle and epidermis) were set between plants long before the advent of large megafilos. Second, during the same interval had an unprecedented explosion of evolutionary innovation in the history of plant life, which witnessed the rise of the trees from ancestors arable , and the evolution of life cycles complexes, including the invention of the seed . Third, the small plant megafilo Eophyllophyton bellum from the early Devonian, shows that plants were able to produce a single megafilo long time before these structures were generalized.

In nearly all existing plants leaves are an adaptation to increase the amount of sunlight that can be captured for photosynthesis. It is accepted that the leaves have evolved more than once in the course of plant evolution and that probably originated as spiny structures that acted as defense structures against herbivores.
The riniópsidas ( Rhyniopsida ) the paleontological site of Rhynie Chert were composed not only by thin lines without any branching. The trimerópsidas ( Trimerophytopsida ) Early Devonian are the first evidence that something is available that can be considered as a leaf. This group of vascular plants are recognizable by their masses of sporangia terminals, which adorn the ends of the axes bifurcate or trifurcate can. 55 Some agencies, such as Psilophyton had growths on the stems, small, spiny, which lacked its own vascular supply. 56 57 About the same time, Zosterophyllopsida were gaining importance. This group is distinguished by its kidney-shaped sporangia growing on short side branches close to the main axes. 55 Most members of this group had sharp spines on their axes. However, such plots lacked leaf spines, and the first evidence of vascular enations occurred in the genus Asteroxylon . The spines of Asteroxylon had a primitive vascular source, since each "leaf" was supplied by a foliar trace that rose from the protoestela. A fossil known as Baragwanathia appears in the fossil record shortly before the end of the Silurian. 58 In this organism, such foliar traces continue into the 'leaves' to form the midrib. 59
Asteroxylon 60 and Baragwanathia club mosses are considered primitive. 55 The club mosses are still currently being Isoetes a well-known example. These current lycopodiums also present with a single trace microphylls foliar and small size. The biggest difference with Lepidodendrales extinct is that they displayed microphylls up to one meter in length, but in almost all cases, there was only one foliar trace. 55
The family leaves, the megafilos , have evolved into four separate occasions and independently: in ferns, horsetails, the progimnospermas and seed plants. 61 seems to have originated from the dichotomization of branches , which first overlapped each other, and eventually developed a "palm" which gradually evolved into leaf-like structures. Then, according to this theory teleoma, the megafilos consist of a group of webbed branches 59 in each of the four groups that were to develop megafilos, leaves first evolved during the Late Devonian to Early Carboniferous, diversifying rapidly to the basic structural designs were established in the Carboniferous medium.

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