It has been known since the work of Piper, Evans, McKee and Morsel that cross-pollination of alfalfa increases the production of seed, as compared with close-pollination. Heretofore, the cause of this behavior has not been revealed. In the present paper it will be shown that the difference is associated with two phenomena, possibly unrelated, whose effects can be distinguished by a histological study of the pistil following the two types of matings. The plant is found to be partially self-incompatible (self-sterile); that is, male gametophytes, while not impotent on the individual from which they arise, are less effective in accomplishing fertilization than are unrelated male gametophytes. The second important reason for the difference in fertility, as measured by the number of functional seeds developed, is the markedly greater tendency for ovules containing inbred embryos rather than those resulting from cross-pollination to collapse during development.The seven plants used in the experiment were the immediate offspring, by close-pollination, of six unrelated individuals selected from commercial varieties of alfalfa. They were grown in a greenhouse at 70-80°F., and the pollinations were made in April, 1938, at the height of the flowering period. Newly opened flowers on several inflorescences on each individual were castrated. Some of these were then close-pollinated by using pollen from other flowers on the same respective plants. The remaining castrated flowers were cross-pollinated, the pollen being derived in each case from a different individual within the group. Each of the plants, therefore, served as a pistillate and as a staminate parent in the crosses, in addition to being selfed. In making the matings, an abundance of pollen was brought into intimate contact with the stigmatic secretion by means of toothpicks tipped with a narrow strip of fine emery cloth. The pistils were collected at 30, 48, 72, 96, 120 and 144 hours after pollination, fixed and embedded in paraffin. After sectioning and staining, the follow-
