Asteroxylon and Baragwanathia

Asteroxylon[42] and Baragwanathia are widely regarded as primitive lycopods.[32] The lycopods are still ‹See Tfd›extant today, familiar as the quillwort Isoetes and the club mosses. Lycopods bear distinctive microphylls – leaves with a single vascular trace. Microphylls could grow to some size – the Lepidodendrales boasted microphylls over a meter in length – but almost all just bear the one vascular bundle. (An exception is the branching Selaginella). The more familiar leaves, megaphylls, are thought to have separate origins – indeed, they appeared four times independently, in the ferns, horsetails, progymnosperms, and seed plants.[43] They appear to have originated from dichotomising branches, which first overlapped (or "overtopped") one another, and eventually developed "webbing" and evolved into gradually more leaf-like structures.[41] So megaphylls, by this "teleome theory", are composed of a group of webbed branches[41] – hence the "leaf gap" left where the leaf's vascular bundle leaves that of the main branch resembles two axes splitting.[41] 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.[43] The cessation of further diversification can be attributed to developmental constraints,[43] 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 b

came significant. However, small, rare mesophylls are known from the early Devonian genus Eophyllophyton – so development could not have been a barrier to their appearance.[44] The best explanation so far incorporates observations that atmospheric CO2 was declining rapidly during this time – falling by around 90% during the Devonian.[45] This corresponded with an increase in stomatal density by 100 times. Stomata allow water to evaporate from leaves, which causes them to curve. It appears that the low stomatal density in the early Devonian meant that evaporation was limited, and leaves would overheat 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.[46] 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.[41] Further, horsetail (Equisetum) leaves bear only a single vein, and appear for all the world to be microphyllous; however, both the fossil record and molecular evidence indicate that their forbears bore leaves with complex venation, and the current state is a result of secondary simplification.