Plant Sciences Department, UC Davis, , Davis, CA, USA, Department of Geological and Environmental Sciences, Stanford University, , Stanford, CA, USA.
Proc Biol Sci. 2014 Jan 29;281(1779):20132829. doi: 10.1098/rspb.2013.2829. Print 2014 Mar 22.
The main role of leaf venation is to supply water across the photosynthetic surface to keep stomata open and allow access to atmospheric CO2 despite evaporative demand. The optimal uniform delivery of water occurs when the distance between veins equals the depth of vein placement within the leaf away from the evaporative surface. As presented here, only angiosperms maintain this anatomical optimum across all leaf thicknesses and different habitats, including sheltered environments where this optimization need not be required. Intriguingly, basal angiosperm lineages tend to be underinvested hydraulically; uniformly high optimization is derived independently in the magnoliids, monocots and core eudicots. Gymnosperms and ferns, including available fossils, are limited by their inability to produce high vein densities. The common association of ferns with shaded humid environments may, in part, be a direct evolutionary consequence of their inability to produce hydraulically optimized leaves. Some gymnosperms do approach optimal vein placement, but only by virtue of their ability to produce thick leaves most appropriate in environments requiring water conservation. Thus, this simple anatomical metric presents an important perspective on the evolution and phylogenetic distribution of plant ecologies and further evidence that the vegetative biology of flowering plants-not just their reproductive biology-is unique.
叶脉的主要作用是在保持气孔开放的情况下将水分输送到光合表面，以允许大气中的 CO2 进入，尽管存在蒸发需求。当叶脉之间的距离等于叶脉在叶片内离蒸发面的深度时，水就能均匀地输送。如这里所展示的，只有被子植物在所有叶片厚度和不同生境中保持这种解剖学上的最佳状态，包括庇护环境，在这些环境中不需要这种优化。有趣的是，基部被子植物谱系在水力方面的投资相对较少；在木兰类植物、单子叶植物和核心真双子叶植物中，这种优化是独立地以高均匀性衍生而来的。裸子植物和蕨类植物，包括现有的化石，受到其无法产生高脉密度的限制。蕨类植物与阴暗潮湿环境的常见关联，可能部分是由于它们无法产生水力优化叶片的直接进化后果。一些裸子植物确实接近最佳的叶脉位置，但这仅仅是因为它们能够产生最适合需要节水的环境的厚叶片。因此，这种简单的解剖学指标提供了一个关于植物生态进化和系统发生分布的重要视角，进一步证明了开花植物的营养生物学——不仅仅是它们的生殖生物学——是独特的。
