MEDICAGO SATIVA 



313 



epicarp is a single layer of elongated cells, except where they form a 

 rosette around a stoma; and epidermal hairs are frequently formed 

 which tend to break off as the fruit dries, leaving a thickened scar. 

 (2.) In the mesocarp, the two important cellular elements are the 

 very thin-walled crystal cells, each of which contains a single 

 crystal; and the fibers which have thick pitted walls and blunt 

 ends. (3) The endocarp con- 

 sists of a single layer of epi- 

 dermal cells in which there are 

 no stomata. 



Pollination. — The varia- 

 tion in the amount of seed 

 produced by a given strain of 

 alfalfa has resulted in many 

 investigations of pollination; 

 and this, in turn, has focused 

 attention on the mechanism of 

 the process. When tripping 

 occurs, due to any of the agen- 

 cies noted below, the staminal 

 column is released and snaps 

 forward until it strikes the 

 standard with considerable 

 force. This must be sufficient, 

 according to Armstrong and 

 White (i), to rupture the cu- 

 ticular membrane which con- 

 stitutes the limiting outer surface of the stigma. The presence of 

 such a limiting membrane was demonstrated byjost (16) in Lupi- 

 nus and Cytisus. Tripping of the stamens may be due to insect 

 visitation; to "automatic tripping" in which some other factor,' 

 possibly climatic, may release the tension; or it may be artificially 

 induced by various agencies. Piper and coworkers (2.5) state that 

 "Atmospheric or climatic conditions greatly affect automatic trip- 

 ping, so that it is not improbable that this factor alone accounts 

 for a great variation in seed production during different seasons." 

 Carlson and Stewart (7) determined that "artificial tripping resulted 

 in an increase of approximately 140 per cent in the percentage of 

 flowers forming buds as compared with natural development"; and, 

 also, that alfalfa may "be expected to seed best under desert or 



Fig. 157. A, habit of inflorescence; B, fruit; 

 C, carpel with other floral parts removed ; D and 

 E, flower, lateral and face views ; F, stamens 

 with perianth removed; G, floral diagram. 



