THE SEED PLANTS 



303 



While these developments have been taking place in the 

 embryo sac, the integument walls have become dry and hard, 

 so that, by the time the young plant has differentiated the 

 root, stem, and leaf regions, it is usually inclosed by hard 

 and dry walls, and the whole structure is a seed. In some 

 cases the seed is dropped 

 from the parent plant at 

 once, and may begin its 

 growth immediately. Seeds 

 may lie dormant on the 

 ground until the return 

 of spring, or may in some 

 cases lie dormant for sev- 

 eral years, and still retain 

 their vitality. The seeds of 

 the cocklebur and of some 

 desert plants may lie in the 

 ground for several years 

 and then grow. The seed 

 coats sometimes become so 

 dry and hard that the water 

 and air necessary for ger- 

 mination cannot penetrate, 

 and this condition remains 

 until, through decay of the 

 wall or through injury to 

 it, the needed materials can 

 get into the seed. Most 

 seeds, such as wheat, corn, 

 and oats, lose their vitality within a few years at the most. 



288. The fruit. The relations between seed and fruit are 

 not easily defined, for the reason that the fruit may include 

 a few or many structures. The ripened ovule containing the 

 embryo isjbhe seed.. Sometimes structures other than the seed 

 ripen with it; regardless of how many things ripen with the 

 seed, all are included in the term fruit. In the sunflower the 



ouint 



FIG. 232. Diagram of the ovule, embryo 



sac, and embryo of the shepherd's-purse 



(Capsella Bursa-pastoris) 



The parts shown are the outer integument 

 (OH int), inner integument (in int), embryo- 

 sac wall (e s w), suspensor cells (s^lsp c), 

 root region (r r), stem region (st r), and 

 seed leaves (s I) 



