56 



SEEDS AND FRUITS 



young plant, which after reaching a certain stage of development, 

 varying in different plants, passes into a dormant stage from which 

 it may awake if conditions are favorable and continue its devel- 

 opment until it becomes a mature plant. In the development of 

 the embryo from the fertilization of the egg to the dormant stage, 

 certain structures which function in the further development of 

 the young plant are usually more or less developed. In a well 

 formed embryo like that of the Bean, there are four parts, hyocotyl, 

 plumule, cotyledons, and radicle. In Figure 60 of the Bean, h is 

 hypocotyl, p, plumule, and c, cotyledons. The radicle (r) is at 

 the lower end of the hypocotyl and is so closely joined with the 



hypocotyl that it does not appear as 

 a separate structure. The cotyledons 

 of the Bean have absorbed the endo- 

 sperm and consequently are so much 

 enlarged that they form the bulk of 

 the embryo. The special functions 

 performed by the different parts of the 

 embryo are quite noticeable in the 

 germination of the seed. The cotyle- 

 dons supply food; the plumule develops 

 stem and leaves; the radicle develops 

 a root; and the hypocotyl in many 

 cases pulls the cotyledons and plumule 

 out of the seed coat and raises them 

 above ground. 



The stored food and seed coat are temporary structures. They 

 nourish and protect the young plant in its early stage of develop- 

 ment and then disappear. The stored food, consisting chiefly of 

 starch, proteins, and oils, the proportion varying in different 

 seeds, develops in close contact with the embryo and when not 

 absorbed as rapidly as it develops, it forms the storage tissue or 

 endosperm in which the embryo becomes imbedded. The testa, 

 the protective structure of the seed and usually formed from 

 the integuments of the ovule, generally consists of a single 

 covering so much thickened and hardened that it protects the 

 embryo against injuries. Often there is a thin inner covering 

 and in exceptional seeds, like those of the Water Lily, an extra 

 outer covering called the aril develops later than the integuments 

 and forms a loose covering about the seed. (Fig. 62.) 



FIG. 60. Bean with testa 

 removed and cotyledons 

 spread apart, c, cotyledons; 

 h, hypocotyl; p, plumule; r, 

 radicle. 



