364 MORPHOLOGY OF THE ANGIOSPERMS 



haustorial organs originating phylogenetically as the nursing foot in the 

 bryophytes and persisting throughout the higher plants." The presence 

 of the scutehum in the embryo of the monocotyledon — a simple, foot- 

 like, absorbing structure, basal in position — was considered evidence 

 of primitiveness. The possession of the scutellum supported the view, 

 held at that time, that the monocotyledons are the primitive angio- 

 sperms and that the grasses, with their large scutellum and "simple" 

 flowers, are among the more primitive families. At the same time, some 

 of the simple aquatic monocotyledons were considered highlv primi- 

 tive, resembling, in habit, some of the pteridophytes, especially Isoetes. 

 (That these aquatic monocotyledons have no scutellum was over- 

 looked.) Recognition of the monocotyledons as highly primitive angio- 

 sperms gave support to the theory that the two cotyledons of the 

 dicotyledons are the morphological equivalent of the one of the mono- 

 cotyledons. 



The cotyledon is surely not homologous with the "foot" of the 

 pteridophytes. Resemblance between these structures is superficial only; 

 both absorb food for embryos. The "foot" is one of the major parts of 

 an embryo; the scutellum is a specialized part of a lateral appendage 

 of the axis. [Furthermore, the scutellum is not the cotyledon; in some 

 species of Peperomia, a dicotvledon, it represents an entire cotyledon 

 (Fig. 136D), but, in monocotyledons, it is onlv the distal part.] In the 

 dicotyledons, at least, the cotyledon is clearly an appendage of the 

 axis of the embryo and, later, of the seedling. Much evidence supports 

 the common view that the cotyledon is a leaf, modified in function as 

 a storage and food-absorbing organ. In details of ontogeny and in 

 vascular anatomy, it is a leaf; and in taxa in which it becomes a photo- 

 synthetic organ, it has typical leaf structure. The interpretation of 

 cotyledons as organs sui generis is based, in part, on the fact that their 

 traces are derived from the hvpocotyl and not from the stem, as are 

 those of leaves, and that usuallv, in position on the axis, cotyledons do 

 not fall into the phyllotactic spirals. But they are lateral appendages of 

 the axis, as are leaves; their traces are derived from the hypocotyl, as 

 leaf traces are derived from the stem. The hypocotyl is transitional in 

 structure from root to stem, and its upper part frequently has typical 

 stem structure. The position of the cotyledons — at the base of the stem 

 — is a feature of the crowding and condensation of the organs in the 

 embryo. In position, form, structure, and ontogeny, the cotyledons are 

 specialized leaves, not organs sui generis. 



It has been argued that the cotyledon is not the equivalent of the 

 leaf, because no leafbase or cushion is formed in its ontogeny. But the 

 existence of leafbases is a controversial subject, and the recognition of 

 them in the embryo would be most difficult, because all parts of the 

 embryo are in the primordial stage and crowded together. 



