Department of Higher Plants, Faculty of Biology, Lomonosov Moscow State University, Moscow 119234, Russia.
Am J Bot. 2013 May;100(5):824-43. doi: 10.3732/ajb.1200620. Epub 2013 Apr 23.
A bipolar embryo with cotyledons is a characteristic feature that appeared early in the evolution of seed plants. Cotyledon number is an important character in angiosperm classification. We explore the links between functional aspects of seed germination and the number and location of the cotyledons, using as a model the early-divergent angiosperm family Hydatellaceae, in which seedlings are superficially monocot-like. •
Seedlings of two species of tropical Hydatellaceae were studied using light and scanning electron microscopy. •
Seedlings of Trithuria cowieana bear two free cotyledons. Each cotyledon possesses a green, filiform, vascularized blade that resembles subsequent leaves, and a basal, nonvascularized, haustorial outgrowth that remains in close contact with the endosperm. Seedlings of Trithuria konkanensis have two free cotyledonary haustoria inserted close to each other and a leaf blade probably belonging to one of the cotyledons. The cotyledonary node elongates between the haustoria and the leaf blade to form a mesocotyl. •
To date, the absence or presence of a cotyledonary tube represents the only known qualitative morphological difference between the two major clades of Hydatellaceae. Cotyledons with a haustorium and leaf blade are unusual at the scale of seed plants and probably evolved due to homeosis. The mesocotyl of T. konkanensis resembles that of grasses and sedges. Seedling diversity in Hydatellaceae and other seed plants is linked with the principal physical and spatial constraint of their embryo structure, with the primary root and shoot apical meristems located at opposite poles, and haustorial cotyledon tips.
具有子叶的二倍体胚胎是种子植物早期进化的一个特征。子叶数量是被子植物分类的一个重要特征。我们使用早期分化的被子植物Hydatellaceae 家族作为模型，探索种子萌发的功能方面与子叶的数量和位置之间的联系，因为该家族的幼苗表面上类似于单子叶植物。
使用光镜和扫描电子显微镜研究了两种热带 Hydatellaceae 物种的幼苗。
Trithuria cowieana 的幼苗有两个自由的子叶。每个子叶都有一个绿色的丝状血管化叶片，类似于随后的叶片，以及一个基部的非血管化吸器突起，它与胚乳保持紧密接触。Trithuria konkanensis 的幼苗有两个自由的子叶吸器彼此靠近插入，并且叶片可能属于其中一个子叶。子叶节在吸器和叶片之间伸长，形成中胚轴。
到目前为止，子叶管的缺失或存在代表了 Hydatellaceae 两个主要分支之间唯一已知的定性形态差异。具有吸器和叶片的子叶在种子植物的范围内是不寻常的，可能是由于同源性而进化的。T. konkanensis 的中胚轴类似于禾本科和莎草科。Hydatellaceae 和其他种子植物的幼苗多样性与它们胚胎结构的主要物理和空间限制有关，主根和茎尖分生组织位于相反的极点，以及吸器子叶尖端。
