DICOTTLEDONES. 



475 



root) ends in a very short root, which is continuous with the stem.* 

 Under the proper conditions of heat and moisture, the root elongates 

 and pushes out through the micro- 

 pyle of the seed-coat ; at the same 

 time, tlie stalks of the cotyledons 

 elongate and thus bring the plumule 

 outside of the seed-coat, the cotyle- 

 dons alone remaining. During the 

 first few days of its growth the 

 young plant is nourished by the 

 starch in the cotyledons, which in 

 this species remain during the whole 

 process of germination beneath the 

 ground enclosed in the seed-coat. In 

 the common Field Bean (Phaseolus) 

 the germination is the same, except- 

 ing that the hypocotyledonary stem 

 elongates, and brings the cotyledons 

 which have slipped out of the seed- 

 coat above the ground. 



The seed of Bicinus (the Castor 

 Oil Plant) contains a large embryo 

 surrounded by a thin layer of endo- 

 sperm (Fig. 368, /). In its germina- 

 tion the root and hypocotyledonary 

 stem elongate, and thus bring the 

 seed-coat with the contained coty- 

 ledons above the ground (Fig. 368, 

 //.). The cotyledons remain within 

 the seed-coat until they have absorb- 

 ed all of the endosperm ; when this 

 is accomplished the empty seed-coat 

 falls away, and the freed cotyledons 

 expand and assume to some extent 

 the function of ordinary foliage 

 leaves. 



The venation of the leaves of Di- 

 cotyledons is easily studied by mac- 

 erating them so as to remove the 

 parenchyma (mesophyll), leaving 

 only the fibro-vascular bundles. 

 While there is as a rule a general 

 likeness between them, there is yet 

 an almost infinite diversity in the 



Fig. 369.— Longitndinal section of the 

 axis of the embryo in the ripe eeed of 

 Phaseolus muUlfioms, parallel to the 

 cotyledons. «s, apex of the stem ; mw, 

 of Ihe root ; ct, swelling near insertion 

 of cotyledons ; i, the first internode ; 

 pb, the petioles of the first foliage 

 leaves ; ^ -y. ij, /, procambiuin of the 

 fibro-vascular bundles ; he, hypocoty- 

 ledonary portion of the stem (the brace 

 is too long in the figure). X 30.— After 

 Sachs. 



* In some old books, and even a few recent ones, a structure called 

 the.collar or collum, is spoken of. Dr. Gray very properly defines it ag 



