GROWING ALFALFA SEED. 449 



because of crowding, the plants are unable to produce seed, ap- 

 parently on account of shading by closely associated individuals. 



In addition to the injurious influence of shade, the crowding 

 of plants interferes with seed production by depriving the plants 

 of sufficient moisture to enable them to mature their seed properly. 

 This, of course, is true only in areas of light rainfall. On the 

 other hand, in sections where irrigation is practiced thick stands 

 by checking evaporation bring about such moist conditions in 

 fields as to promote unfavorable conditions and so prevent maxi- 

 mum yields of seed. 



The basal shoots which usually appear when the plant begins to 

 bloom are developed at the expense of the seed crop. The energy 

 that should be devoted solely to the maturing of the seed is di- 

 verted by this new growth. Perhaps the most important factor 

 influencing the development of these basal shoots which are to 

 form the succeeding crop is the water content of the soil. If the 

 moisture supply be ample, the basal shoots commence their growth 

 about the time the plant comes into bloom. This is disastrous to 

 the seed crop, and for this reason it is necessary that there be a 

 sufficient shortage of moisture at this time to retard or prevent 

 altogether the development of these shoots. In the seed-produc- 

 ing sections of the more humid parts of the Great Plains area 

 profitable crops of alfalfa seed are usually obtained only in the 

 occasional seasons of drought so extreme that the yield of other 

 crops is greatly reduced. 



Drought is used here in a qualified sense. There must of course 

 be enough moisture in the soil to enable the seed to mature fully; 

 otherwise it will be deficient in germinating power. On the other 

 hand, the soil must not contain enough moisture to force into 

 growth the crown buds that produce the succeeding crop. 



The favorable conditions for the production of alfalfa seed 

 which prevail in the semi-arid regions are due principally to the 

 presence there of a favorable adjustment of the supply of moist- 

 ure in the soil to the moisture requirements of the plant when 

 grown for seed. This is especially true when the plants are grown 

 in cultivated rows, as the moisture content of the soil can then 

 be regulated to some degree by proper cultivation. 



Insects and the Setting of Seed. Insect visits are essential to 

 the proper pollination of the alfalfa flower. If fertile seed is to 

 be produced in any quantity it is necessary that a certain ex- 

 plosive mechanism within the flower be released. The release of 

 this mechanism, whether it be accomplished by insects or other- 

 wise, is popularly called tripping. 



Experiments and observations both by the writers and by other 



