DIFFERENCES IN STRUCTURES OF SEEDS 



77 



different plant of the same or a closely related species or variety, 

 the new plants shonld possess some characters of both parents, 

 This gives a basis for work in the production of new varieties of 

 plants. (Chapter XV.) 



Differences in Structures of Seeds. — There are some facts 

 concerning the structure of the seed which 

 we have already learned, but which we 

 should now recall in the light of these new 

 facts. (Chapter VIL) The young plant 

 or embryo consists of a very short. stem on 

 one end of which is a root tip ; and on the 

 other end a plumule, or bud, and one or two 

 cotyledons or primary leaves. It is a com- 

 plete plant possessing the three essential 

 parts: root, stem, and leaf. We will recall 

 that the seeds of corn and castor oil plants 

 contained embryos surrounded by an abund- 

 ant food supply for their nourishment dur- 

 ing the period of germination and before 

 they had become firmly established as inde- 

 pendent plants. We will also recall that in 

 the bean we found a much larger embryo 

 without the surrounding food supply. In this case the food for 

 the early growth of the young plant was in the cotyledons. 



The seeds of corn and castor oil plant matured very early 

 when the embryos were small, but with a good supply of stored 

 food for the growth of the embryo during germination. The 

 seed of the bean matured much later, the embryo having ab- 

 sorbed the surrounding food supply and stored it in the coty- 

 ledons previous to maturity. In the corn, the single cotyledon 

 is an organ which serves for the absorption of the food supply; 

 in the castor oil plant the cotyledons serve first as organs of 

 absorption and later as the primary leaves; in the bean they 

 serve for the storage of the food supply and as primary leaves. 



Fig. 60. — Diagram- 

 matic longitudinal sec- 

 tion of ovary showing 

 developing ovules. 



