348 FORAGE PLANTS AND THEIR CULTURE 



involved are obscure. Maximum yields of 20 bushels 

 to the acre have been reported, but 8 bushels is considered 

 a large yield. Returns of from 2 to 5 bushels an acre 

 represent the usual crop. In Europe the yield to the 

 acre ranges from 300 to 700 pounds, according to different 

 authorities. 



In recent years, an increasing amount of alfalfa seed 

 has been grown on unirri gated semi-arid lands. Such 

 seed is considered preferable for dry land farming, and 

 this is probably so, but there is no convincing experimental 

 evidence of such superiority. In growing alfalfa for seed 

 production on dry land, very thin stands are best, but 

 there is an increasing tendency to plant it in rows about 

 three feet wide with the plants about one foot apart in 

 the rows. This permits of cultivation as frequently as 

 may be desired. 



The crop, if harvested for seed, should be cut as soon 

 as most of the pods are ripe and the seeds yellow and 

 hardened. 



413. Pollination. The structure of the alfalfa flower 

 has a peculiar explosive mechanism which especially 

 adapts it to being cross-pollinated by large bees, especially 

 bumble bees. The filaments of the upper stamens form- 

 ing the stamineal tube are under tension, but are held in 

 a straight position in the keel by means of processes on 

 the wings. The insertion of a toothpick into the nectary 

 or the pressing downward of the keel will release these 

 processes, when the stamineal column with the inclosed 

 pistil recurves, violently striking the standard. This 

 process is called " tripping." When a bee trips a flower, 

 the pollen is scattered on its under side. If it then visits 

 and trips another flower, it is quite likely to dust pollen 

 from the first on the stigma of the second. 



