MEDICAGO SATIVA 315 



and, prior to dehiscence, tlie nucleus divides to form two daughter 

 nuclei of unequal size. 



Dehiscence of the anthers occurs in the bud stage and the liber- 

 ated pollen is confined to the keel, closely packed around and 

 directly on the stigma. This method of pollen release leads to self- 

 fertilization in most instances, but there is some cross-pollination. 

 It has been noted that pollen frequently occurs on the standards of 

 untripped flowers in the field, and such grains, which have been 

 transmitted by insects or other agencies, provide an opportunity for 

 cross-fertilization. It has been demonstrated further that cross- 

 fertilization greatly enhances the production of seed, and Carlson 

 (6) reports "that 17 per cent of alfalfa flowers allowed to develop 

 naturally formed seed pods, while 44 per cent of those tripped arti- 

 ficially and 54 per cent of those artificially cross-pollinated formed 

 seed pods." 



The Development of the Ovule and Megasporogenesis. — The 

 ovules arise from parietal placentae in two rows which are so 

 crowded that transections of the carpel may show two ovules 

 apparently side by side, but as the carpel lengthens rapidly, they 

 become separated and appear to be in one row. The young ovule 

 is erect, beginning to curve at about the time that it comes in 

 contact with the dorsal wall of the carpel. Reeves Qvf) observed 

 that the curvature is usually toward the basal portion of the ovary; 

 although, occasionally, one of the ovules at the stylar end may 

 curve toward its apex. (Fig. 158, A.^ At maturity, the ovule is 

 campylotropous with the micropyle resting against the funiculus. 

 Two integuments develop, each of which is two cell layers in thick- 

 ness, except near the micropyle where the outer integument is 

 considerably thicker. In the development of the ovule, the inner 

 integument arises first, but the outer one grows more rapidly and 

 soon encloses it. Since the inner integument does not usually 

 completely surround the nucellus, a considerable part of the 

 nucellus adjacent to the micropyle is covered only by the outer 

 integument. (Fig. 158, B and C.) As the ovary develops, the 

 carpel itself becomes coiled, the curvature being greater at its apical 

 end. 



The number of archesporial cells may vary. Martin (19) 

 observed a multiple archesporium from which primary parietal and 

 primary sporogenous cells are derived; and both Reeves (xy) and 

 Cooper (9) have noted that there are usually two or three primary 



