210 



PLANT STRUCTURES 



ledons often organizes the rudiments of subsequent leaves, 

 forming a little bud which is called the plumule. 



Embryos differ much as to com- 

 pleteness of their development within 

 the seed. In some plants, especially 

 those which are parasitic or sapro- 

 phytic, the embryo is merely a small 

 mass of cells, without any organiza- 

 tion of root, stem, or leaf. In many 

 cases the embryo becomes highly de- 

 veloped, the endosperm being used 

 up and the cotyledons stuffed with 

 food material, the plumule contain- 

 ing several well - organized young 

 leaves, and the embryo completely 

 filling the seed cavity. The com- 

 mon bean is a good illustration of 

 this last case, the whole seed within 

 the integument consisting of the two 

 large, fleshy cotyledons, between 

 which lie the hypocotyl and a plu- 

 mule of several leaves. 



115. The seed. — As in Gymno- 

 sperms, while the processes above 

 described are taking place within 

 the ovule, the tissue is developing 

 that forms the hard seed-coat or testa (Fig. 183). When 

 this hard coat is fully developed, the activities within 

 cease, and the whole structure passes into that condition of 

 suspended animation which is so little understood, and 

 which may continue for a long time. 



The testa is variously developed in seeds, sometimes 

 being smooth and glistening, sometimes pitted, sometimes 

 rough with warts or ridges. Sometimes prominent append- 

 ages are produced which assist in seed-dispersal, as the 

 wings in Catalpa or Bignonia (Fig. 184), or the tufts of 



Fig. 182. Young embryo of 

 water plantain {Alisma), a 

 Monocotyledon, the root 

 being organized at one 

 end (next the suspensor), 

 the single cotyledon (C) 

 at the other, and the stem- 

 tip arising from a lateral 

 notch (v). — After Han- 

 stein. 



