330 MORPHOLOGY OF THE ANGIOSPERMS 



Sanguisorba. This tube may become much elongated and petioleUke — 

 Thea, Quercus — superficially resembling the "neck" of the single 

 cotyledon in many monocotyledons. 



The form of the cotyledon, especially the position of the scutellum, 

 varies greatly, even within a family — Liliaceae, Palmae, Gramineae 

 (Fig. 123). 



In the specialization of the cotyledon, its modification in adap- 

 tation to change of function, from food storage to absorption of food 

 from endosperm and perisperm, and to protection of the plumule dur- 

 ing germination (coleoptile) has been varied and, structurally, very 

 great (Fig. 130). The modifications are related largely to evolutionary 

 changes in type of germination associated primarily with the develop- 

 ment of storage endosperm and perisperm. Hypogeal germination 

 doubtless represents an advance over epigeal germination. Transitions 

 from epigeal to hypogeal germination are well shown by liliaceous 

 genera, especially the Scilleae and Lilium, where both types occur. 



The change from epigeal to hypogeal germination and its accompany- 

 ing morphological modifications have occurred in both dicotyledons and 

 monocotyledons but are more general and much greater in the mono- 

 cotyledons. No line exists between the types, and the change is ob- 

 viously an easy step, which has been taken many times. A family or 

 genus may possess only one type, or both types, with transitional forms 

 — Lilium, Sophora, Erijthrina, Phaseolus. Probably in only a few dicoty- 

 ledons has the absorbing function developed in the cotyledon. In 

 Feperomia, the entire lamina of one cotyledon has become suctorial 

 (Fig. 136); in the monocotyledons, a distal part or all of the lamina is 

 transformed. 



The structural changes — step by step — in the elaboration of the suc- 

 torial habit are well illustrated in the monocotyledons; the tip of the 

 cotyledon is first modified, then the entire lamina is transformed into a 

 specialized absorbing structure, ultimately retained permanently in the 

 seed coats. The absorbing part of the cotyledon, in its most specialized 

 form, is the scutellum. The change related to the protection of the 

 embryonic bud is the transformation of a median part of the cotyledon 

 into a caplike cover for the plumule, the coleoptile (Fig. 130). The 

 distal and proximal parts of the cotyledon are separated by a long, 

 slender median part, the "neck," which is prominent in many mono- 

 cotyledons. This neck {"Zwlschenstilck" of some early authors) was 

 generally recognized in the nineteenth century as a part of the cotyle- 

 don, connecting the two specialized ends. (The extended neck of the 

 cotyledon is clearly correlated with hypogeal germination.) 



In the highest grasses, the top of the neck becomes adnate to the 

 hypocotyl and is no longer externally evident, except in those taxa in 



