374 



A TEXTBOOK OF BOTANY [Ch. VIII, 2 



Fig. 267. — Embryo 

 of Lima Bean, as it 

 appears without the 

 seed coat, and on re- 

 moval of one cotyle- 

 don ; X 1- The largest 

 part is cotyledon. 



time in suspension. It has always a small unjointed stem, 

 the hypocotyl, which bears at one end the foundation for 

 a root, at the other the foundation for a bud, and close 

 to the latter one or more "seed leaves," 

 or cotyledons fFigs. 267-9). These 

 cotyledons, which vary from leaf-thin to 

 hemispherically thick, are oftenest two, 

 less frequently one, and sometimes 

 several, as prevails in the Pine family. 

 While the cotyledons are commonly 

 viewed as morphologically leaves, modi- 

 fied by their peculiar position and func- 

 tions, there is doubt as to whether they 

 really originated as leaves, or are the 

 descendants of special organs by which 

 embryos originally absorbed food direct 

 from the parent plants. The bud be- 

 tween the cotyledons is mostly undeveloped in the seed, 

 but in some large, well-developed embryos, it produces visible 

 leaves, in which case it is called the plumule (Fig. 267). 



Second in importance is the nutritive food, which is sup- 

 plied by the parent plant, and consists chiefly of starch, oils, 

 and proteins, in a dry and concentrated condition. In some 

 kinds this food occurs in a special tissue, 

 called endosperm, surrounding the em- 

 bryo, which therefore lies embedded 

 within it (the so-called albuminous 

 seeds, Fig. 270) ; but in others the food 

 is contained inside the embryo itself, f \l G - 268 - — Embryo 



J 7 of Morning Glory, with 



mostly in the cotyledons which are then one cotyledon removed; 

 conspicuously enlarged ; and the embryo x lu ^ ule There 1S no 

 now completely fills the space within the 

 seed coats (ex- albuminous seeds, Fig. 271). The endosperm 

 originates in the embryo sac simultaneously with the embryo 

 (page 354), and the two develop step by step together until 

 they fill the embryo sac, and even (through the absorption 



