THE EMBRYO 



329 



in Peperomki; the cotyledon, like the leaf, is not fundamentally peltate. 

 The true peltate leaf is a specialized form, developed by the union of 

 basal lobes of the lamina. 



The cotyledon is a simple organ in most of the dicotyledons (Fig. 

 127) and in the more primitive monocotyledons (Fig. 135). In the 

 more advanced monocotyledons where, in specialization, 

 it has assumed the functions of absorption of stored food 

 (the scutellum) and of specialized protection for the 

 plumule (the coleoptile), it is a complex organ, no 

 longer recognizable as a simple lateral appendage. In its 

 most extreme form — ti'ansverse division into two segre- 

 gated parts, serving different functions — it is one of the 

 most highly modified plant organs (Fig. 130). 



The cotyledon is typically simple in shape, laminar, 

 conforming in most exalbuminous seeds more or less to 

 the shape of the seed. It may strongly resemble the leaf 

 and, after germination, function similarly, as in many of 

 the more primitive monocotyledons. Where the seed is 

 exalbuminous, or has very little endosperm, the cotyle- 

 don usually has much the form of the normal leaf and, 

 when developed after germination, closely resembles the 

 leaf — Alismataceae, Butomaceae, Triglochin, Philydrum, 

 Orontium, Alliian, Trillium, Tofieldia, Narthecium. The 

 cotyledon may persist as a photosynthetic organ through- 

 out the first growing season; in some monocotyledons, 

 the cotyledon, or part of it, serves as the only assim- 

 ilatory organ for the first year — Eri/fhronium and other 

 liliaceous genera. When not leaflike at germination, it cotyledons, plu- 

 may enlarge greatly and take on leaf structure and func- ^d^'root"^apex' 

 tion, as in many dicotyledons and some of the more 

 primitive monocotyledons — Liliaceae (especially the Scilleae), Araceae. 

 Rarely, it is leaflike in form in the seed, as in some Dioscoreaceae, 

 Euphorbiaceae. 



Where serving primarily as storage organs, cotyledons are thick and 

 often distorted in shape. They are bilobed, even divided, in some 

 dicotyledonous families — Cruciferae, Tiliaceae, Juglandaceae, some of 

 the Amentiferae. The cotyledon of the monocotyledons is typically 

 elongate, with a sheathing base which is closed for some distance above 

 the base. (The plumule is enclosed by this sheathing base, with, often, 

 only a small, slitlike opening above its tip. The developing plumule may 

 extend through this opening or break through the sheath, as in some of 

 the palms and lilies.) In the dicotyledons, the bases of the cotyledons 

 frequently fuse, forming a shallow cup or even a tube — Podophyllum, 



Fig. 127. Dia- 

 gram of longi- 

 tudinal section 

 of mature di- 

 cotyledonous 

 embryo, Pyrus 

 Mains, showing 



