158 



INTEODUCTION TO BOTANY 



The internal structure of seeds differs greatly in the vari- 

 ous kinds. Some contain no separate parts that can be readily 

 made out. Many kinds, however, consist of 



1. An embryo, or miniature plant. 



2. Some plant food stored elsewhere than in 

 the embryo. This is usually called endosperm. 1 



3. A seed coat or coats. 



Frequently the embryo is found to have a 

 fairly well-defined set of organs: the hypocotyL 

 or little stem ; the cotyledons, or seed leaves ; 

 and the plumule, or seed bud. 



149. Classification according to number 

 of cotyledons. The seeds of one great divi- 

 sion of seed plants, the monocotyledons, com- 

 prising grasses, sedges, palms, lilies, and 

 many other groups, have one cotyledon. As 

 shown in figure 158, 

 B, the reserve food is 

 stored mainly outside 

 the embryo. 



The seeds of the 

 other and still larger 

 division, the dicotyle- 

 dons, have two cotyle- 

 dons (figs. 141 and 

 144). The plant food 

 in the seeds of dicoty- 

 ledons is often stored 



in the embryo itself (figs. 140 and 141), 



as in the chestnut, hazel, beech, oak, 



pea, bean, squash, and sunflower ; or 



often between or around the cotyledons of the embryo, as 



in the buckwheat, four-o'clock, castor bean, honey locust, and 



morning-glory (fig. 144). 



1 Reserve food that was formed outside of the embryo .sac is called 

 perisperm. 



-P 



'-hyp 



FIG. 140. Length- 

 wise section of 

 squash seed 



hi, hilum, or scar, 

 marking place of 

 attachment to the 

 ovary ; hyp, hypo- 

 cotyl ; p, plumule ; 

 c, cotyledon ; e (in- 

 nermost layer next 

 to cotyledon), en- 

 dosperm ; t, testa. 

 Two and one-half 

 times natural size 



FIG. 141. A common bean 



split open, after soaking 



in water 



h, hypocotyl, lying on one of 

 the cotyledons; g, groove in 

 the other cotyledon, where 

 the hypocotyl lay; p, plumule 



