THE EMBRYO 313 



sperms, and the endosperm develops around it. Within the angiosperms, 

 the suspensor seems to show reduction from a massive, elongate struc- 

 ture to a few-celled filament or a single cell. Suspensors consisting of 

 only one or two cells appear to be vestigial structures and function only 

 in minor degree. They have often been overlooked, and the embryo 

 described as without a suspensor. 



In angiosperm embryos that have the greatest reduction of the sus- 

 pensor, all, or nearly all, the tissues formed from the zygote go into 

 the formation of the mature embryo. In the gymnosperms, a major part 

 of the young embryo is tissue accessory to the embryo proper: massive, 

 multicellular suspensors and, in some taxa — Araucariaceae, Welwitsch- 

 iaceae, some of the Podocarpaceae and Taxaceae — protective caps 

 about the embryo initials. In the angiosperms, the suspensors are mostly 

 few-celled, and there is no cap. 



Later Stages of Embryo Development. In early stages, the embryo is 

 more or less clearly separable into suspensor and embryo proper. The 

 first divisions commonly form a filament of cells which differentiates 

 into a basal part, the suspensor, and a terminal, early-enlarging part, 

 the embryo proper, from which the body of the mature embryo forms 

 (Fig. 117). The suspensor varies greatly in size and form in different 

 taxa — even in a single family, as in the orchids. It may consist of a 

 single cell, a chain of cells, or a mass of cells not readily separable 

 from the embryo proper. The embryo proper (Fig. 118B) develops, 

 typically, by divisions of the apical, or apical and subapical, cells to form 

 quadrants and octants (Figs. 119 and 120). By continuing division in 

 many planes, there is formed a multicellular sti'ucture, which varies 

 greatly in form in different taxa — pyriform, subspherical, ovoid, obovoid, 

 columnar, flattened. On this undifferentiated mass, the suspensor often 

 persists as a minor appendage (Figs. 120N and 121P). In some taxa, as 

 in the Nymphaeaceae, the suspensor may develop late and has been 

 called vestigial. The single-cell suspensor (Fig. 120) has also been 

 considered a remnant, but functions by great enlargement. Some 

 embryos have been described as having no suspensors, but, in in- 

 terpretations of embryos in which only one or two basal cells are 

 formed by the first transverse divisions in tlie proembryo (Fig. 119), 

 these cells are not considered a suspensor, especially in many of the 

 earlier descriptions of embryo development. 



Differentiation of organs of the embryo — cotyledons and axis — may 

 be immediate, brought about by continuous growth of the embryo, or a 

 period of cessation of growth may intervene before the final stages of 

 development. This interruption in embryo development may play a 

 part — largely unrecognized — in the process of oftcr-ripcning of seeds. 



As compared with the great number of descriptions of embryo ( chiefly 



