86 



AGRONOMY 



FIG. 76. External view of 

 lima bean 



mic, micropyle ; hil, hilum ; 

 tes, testa 



catch into the clothing of man and the other animals ; while 

 not a few shoot their seeds for some distance or in other 



ways provide for their dispersal. 



The seed. The seed consists of an 

 outer covering, called the testa, within 

 which is a young plant, or embryo. The 

 testa is marked externally by a scar, 

 the hilum, where the seed was attached 

 to the parent plant. Near the hilum is 

 a tiny opening through the testa called 

 the micropyle. The embryo always 

 consists of a stemlike part, the cauli- 

 cle, to which are attached one or two seed leaves, or cotyledons. 

 In most cases a tuft of very rudi- 

 mentary leaves, a bud in fact, is found 

 at one end of the caulicle. This is 

 the plumule. The embryo is always 

 provided with a food store sufficient 

 to give it a start in life. In plants 

 like the bean this food supply may 

 be stored in the young plant or it 

 may be stored within the testa but 

 outside the embryo, as in the castor 

 bean, in which case it is known as the endosperm, or albumen. 

 So unvarying is the occurrence of either one 

 or two cotyledons in each kind of seed that this 

 fact is commonly seized upon to divide the 

 world of flowering plants into two groups. The 

 plants whose seeds contain only one cotyledon 

 are called monocotyledons, and those whose seeds 

 contain two are called dicotyledons. The differ- 

 ences between the groups, as we have seen, are 

 not confined to the cotyledons alone, but are manifested in 

 all the conspicuous parts of the plant. 



-tes 



FIG. 77. Embryo of lima bean 



plu, plumule; can, caulicle; 

 cot, cotyledon; tes, testa 



FIG. 78. Seed 

 of the castor 

 bean, showing 

 the projecting 

 caruncle 



