488 THE STRUCTURE OF ECONOMIC PLANTS 



artificial cross-pollination to permit the growing of seedlings. 

 Experimental breeding has been carried on at the Virgin Islands 

 since 192.1, and a large number of seedlings have reached maturity. 

 In 192-5, the Virgin Islands Experiment Station reported that 

 approximately 140 varieties had been grown in the preceding 

 three-year period. 



Thompson (13) describes the flower as follows: 



"Usually, the flowers measure from ^^ to 13.2 inches in diameter, and 

 the tube i to 1^2 inches in length. The color varies somewhat in the 

 different varieties and, except in rare instances, is a shade of red, 

 turning darker in the throat of the blossom. In iioo seedlings examined 

 there were two exceptions to this rule . . . bearing pure white flowers. 

 Five stamens of varying lengths encircle the pistil and are attached 

 to the inner surface of the corolla tube, near its base. The two longer 

 ones either meet at or extend beyond the stigma, thus facilitating self- 

 pollination. The pollen grain is spherical and bears many minute 

 papillae which are symmetrically arranged upon its surface. The grain 

 is 0.09 to 0.1 mm. in diameter. . . . The seeds are borne in a seed 

 pod which is similar to that of the morning-glory, and range from 

 I to 4 in a pod." 



Natural crossing occurs readily and is probably carried on by 

 hymenopterous insects, the most important of which is the honey 

 bee. According to Thompson: 



"The sweet-potato blossom opens only once. The bud opens during 

 the night and remains open during the morning hours, then the corolla 

 closes and withers. The time of closing of the blossom varies some- 

 what with the character of the day and the season. The flowers may 

 remain open all day during cool, cloudy weather, especially in January 

 and February, but they ordinarily close about noon on bright, warm 

 days during this season. ... In the latter part of April, closing of 

 the flowers in the field on clear, warm mornings was observed to take 

 place between 9 and 10 o'clock." 



The Fruit and Seed. — Under conditions in the United States, 

 the sweet potato rarely produces viable seeds. Stout (11) con- 

 cluded that sterility has been accentuated through the perpetua- 

 tion of varieties having sterile tendencies for more than 400 years 

 of vegetative propagation; that blooming requires a longer grow- 

 ing season than is necessary for the development of a crop of roots; 

 and that some type of self-sterility or incompatibility operates to 

 limit seed production even when environmental conditions favor 

 blooming. 



