SKCT. I 



MOKPHOLOGY 



167 



dernicitogeii, periblem, and plerome are employed merely for con- 

 venience to designate certain cell layers of the growing point, and are 

 not to be regarded as significant of any peculiar histogenetic or tissue- 

 forming ability. The external layer from which the epidermis 

 develops usually remains a single cell layer. The rudiments of the 

 still undeveloped vascular bundles soon appear in the central cylinder 

 as procambium strands ; while the 

 endodermis of roots is derived at an 

 early stage from the innermost layer 

 of the cortex. 



The stage of the ontogeny of the 

 plant known as germination is 

 specially defined in Phanerogams, 

 where the term is limited to the 

 further growth of the embryo already 

 present within the seed. The embryo, 

 enclosed within the seed, by the time 



Fio. 172. — Thuja occidentalis. ^, Mediau longitudinal 

 section through the ripe seed ( x 5). Z>, C ( x 2) ; 

 D, E (nat. size), different stages of germination ; 

 h, hyiwcotyl ; c, cotyledons ; r, radicle ; v, vegeta- 

 tive cone of stem. 



Fig. 173. —Seedling of Carpinus Betulus. 

 h, Hypoeotyl ; c, cotyledons ; hw, 

 main root ; siv, lateral roots ; r, root- 

 hairs ; e, epicotyl ; I, I', foliage leaves. 

 (Xat. size.) 



the latter is shed from the parent plant generally exhibits the segmenta- 

 tion characteristic of Cormophytes. Protected by the hard seed-coats, 

 it is enabled to sustain a long period of rest. Abundant deposits of 

 nutritive material in the embryo itself, or surrounding it, are provided 

 for its nourishment during germination. The different segments of 

 a phanerogamic embryo have received distinctive names ; thus, as in 

 the embryo of the American Arbor Vitae (Thuja occidentalis, Fig. 172), 

 the stem portion {h) is termed the hypocotyl, the first leaves (c) are 

 the SEED LEAVES or COTYLLDONS, while the root (r) is distinguished 



