DEVELOPMENT OF THE SEEDLING PLANT. 



593 



the so-called tufted stem are illustrated by the germination of the Cow- 

 slip as observed by Mr. Holland. The seedling (tig. 597, A) germinates 

 in the usual way ; but, after a time, the weight of the rapidly -growing 

 plumule causes the tigellum to bend downwards and become more or 

 less horizontal. Adventitious roots are then thrown out from the top of 

 the original tigellum, which gradually decays away, and the seedling 

 stage is completed. 



The size and appearance of the tigellum vary very much : sometimes 

 it constitutes nearly the whole of the embryo; at other times it is 

 at first relatively small, but is afterwards dilated into tuberous root- 

 stocks and similar formations. 



Fig. 597. 



Germination of Cowslip A . First stage with radicle, tigellum, two leafy stalked 

 cotyledons, and plumule. B. Second stage. (See text.) 



Monocotyledonous Germination. The radicle protrudes through 

 a coleorhiza or root-sheath. Usually it speedily ceases to grow, its 

 place and functions being assumed by numerous secondary or 

 adventitious roots, which also burst through a root-sheath, whence 

 Monocotyledons are sometimes called endorhizal, in contradistinc- 

 tion to Dicotyledons, which are exorhizal. Some of the latter plants, 

 however, e. g. Tropceolum, have a distinct root-sheath. The single 

 cotyledon is either applied to the side of the perispenn in the 

 form of a shield-like body, as in most Grasses, or it is enclosed within 



2Q 



