ANGIOSPEEMS 



327 



sir 



300. The embryo and seed. After its formation the oospore 

 at once proceeds to divide and to form the new plant. First 

 it divides in such a way as to form a suspensor, which usually 

 attaches the embryo to the wall of the embryo sac (Fig. 267). 

 The other end of the embryo differentiates a root tip, a stem 

 tip, and one or two (some- 

 times several) leaf tips. ^^ ~^\ ^s / 

 Sometimes there is but 

 one leaf tip, which grows 

 from the end opposite the 

 root tip, while the stem 

 is laterally placed. This 

 is true in those plants 

 which are called monocoty- 

 ledons (meaning " one seed 

 leaf"). In other plants, 

 the dicotyledons (meaning 

 "two seed leaves"), the 

 two (rarely more) seed 

 leaves arise laterally and 

 the stem tip is terminal. 



After these structures 

 have been formed the in- 

 tegument walls become 

 dry and hard, and the seed 

 is completed. It may be 

 shed and grow at once, or 

 it may lie upon the ground 

 until the next year. Some 

 seeds lose their vitality soon after they are formed. In other 

 cases (cocklebur and some desert plants) they may lie m the 

 ground for one to several years and then grow. Some seeds 

 may be kept for several years (wheat, corn, etc.) and retain 

 in part or entirely their ability to germinate, but usually they 

 lose their vitality after a few years at the most. The relations 

 between seed and fruit are difficult to define. The ripened 



suspc 



m mt 



- ou int 



!FiG. 267. Diagram of the ovule, em- 

 bryo sac, and embryo of shepherd's-purse 

 [Capsella) 



The parts shown are the outer integument 

 (ouint), inner integument (in int) , embryo 

 sac wall (esw), suspensor cells (susp c) , root 

 region (rr), stem region {str), and seed 

 leaves (« I) 



