446 BOTANY OF CROP PLANTS 



of pods set and the number of seeds per pod are increased 

 if cross-pollination (xenogamy) is accomplished. 



Factors Affecting Seed Production. — As has been indi- 

 cated, cross-pollination results in a greater crop of seed than 

 self-pollination. An abundance of tripping insects may 

 increase considerably the seed output; however, good seed 

 crops occur in regions where tripping insects are scarce. 

 Seed production is usually Hght in humid sections of the 

 coimtry. Moreover, too much irrigation water appHed 

 during the flowering period is detrimental to seed production. 

 The heaviest yields of alfalfa seed occur in the arid sections 

 of Kansas, Colorado, Utah and Idaho. Isolated plants 

 invariably produce a greater crop of seed than those in a 

 thick stand. The sun's heat favors automatic tripping. 



Martin finds that the setting of seed pods in alfalfa is 

 largely dependent upon the proper functioning of the pollen. 

 The pollen grains require a certain amount of water to germi- 

 nate. When a pollen grain comes to the stigma, the amount 

 of water it finds there depends upon the moisture delivery 

 of the stigma and the moisture of the air. The supply of 

 water for germination of the pollen grains may be changed 

 by increasing the water in the soil, or the atmospheric 

 humidity about the plant. 



Fruit. — This is an indehiscent legume, coiled two or three 

 times (Fig. 187). There are one to eight seeds in each pod; 

 they are kidney-shaped, and about ^ inch long. The seeds 

 retain their viabihty for many years. 



Germination and Seedling.^ — The young seedling consists 

 of two short cotyledon?, a hypocotyl, and a tap root. The 

 first foHage leaf is simple, while the second, third, and all 

 others are trifoliate. There is soon formed an erect stem 

 with but few branches; hence the first growth looks thin. 

 However, there spring up later numerous branches from 



