Beerling D J, Osborne C P, Chaloner W G
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
Nature. 2001 Mar 15;410(6826):352-4. doi: 10.1038/35066546.
The widespread appearance of megaphyll leaves, with their branched veins and planate form, did not occur until the close of the Devonian period at about 360 Myr ago. This happened about 40 Myr after simple leafless vascular plants first colonized the land in the Late Silurian/Early Devonian, but the reason for the slow emergence of this common feature of present-day plants is presently unresolved. Here we show, in a series of quantitative analyses using fossil leaf characters and biophysical principles, that the delay was causally linked with a 90% drop in atmospheric pCO2 during the Late Palaeozoic era. In contrast to simulations for a typical Early Devonian land plant, possessing few stomata on leafless stems, those for a planate leaf with the same stomatal characteristics indicate that it would have suffered lethal overheating, because of greater interception of solar energy and low transpiration. When planate leaves first appeared in the Late Devonian and subsequently diversified in the Carboniferous period, they possessed substantially higher stomatal densities. This observation is consistent with the effects of the pCO2 on stomatal development and suggests that the evolution of planate leaves could only have occurred after an increase in stomatal density, allowing higher transpiration rates that were sufficient to maintain cool and viable leaf temperatures.
大型叶具有分支叶脉和平展形态，其广泛出现直到约3.6亿年前泥盆纪末期才发生。这一事件发生在简单的无叶维管植物于志留纪晚期/泥盆纪早期首次在陆地定殖约4000万年后，但当今植物这一常见特征出现缓慢的原因目前仍未得到解决。在此，我们通过一系列使用化石叶特征和生物物理原理的定量分析表明，这种延迟与古生代晚期大气pCO₂下降90%存在因果联系。与对典型的泥盆纪早期陆地植物（无叶茎上气孔很少）的模拟不同，对具有相同气孔特征的平展叶的模拟表明，由于太阳能截留增加和蒸腾作用低，它会遭受致命的过热。当平展叶在泥盆纪晚期首次出现并随后在石炭纪多样化时，它们具有显著更高的气孔密度。这一观察结果与pCO₂对气孔发育的影响一致，并表明平展叶的进化只能在气孔密度增加之后发生，从而允许更高的蒸腾速率，足以维持叶片凉爽且存活的温度。
