1 66 



during the preceding winter did not produce a single flower. 

 Two plants, out of fifteen, of another clone bloomed: one ga\'e 

 six flowers from Dec. 21, 1923 until Jan. 25, 1924 (see Figure 3), 

 the other yielded five flowers from Feb. ist to March 3rd. With 

 one or two exceptions the anthers in all the flowers were fulh- de- 

 veloped and dehiscence was perfect. Pollen was abundant and 

 under the microscope appeared plump and fully developed. Eight 

 flowers were carefully self-pollinated, two opened and shed pollen 

 by themselves, one flower was crossed using pollen of an Ipomoea 

 blooming in the greenhouse at the same date. In ever>' case 

 the ovary promptly shrivelled and fell with no enlargement 

 whatever. 



During these two years of experience there were only three 

 plants of two clones in bloom and there was no chance to make 

 cross-pollinations between clones. 



Discussion and Conclusion 



The sweet potato has been propagated almost exclusively by 

 vegetative means for a period of at least 400 years and in the 

 more recent >ears its cultivation has been rather extensive. 

 Also it is only the purely vegetative organs (the fleshy roots) 

 that are used as food. The varieties have not been subjected 

 to any sort of selection for production of flowers, fruit, or seed, 

 as have many kinds of cultivated plants. 



These conditions have given full opportunity for the persistence 

 of any types of sterility which may have existed in the original 

 seedlings from which the clonal varieties now in cultivation 

 have been derived or of any types of sterility which may have 

 developed later. It is not, therefore, surprising that the vari- 

 eties of the sweet potato in cuIti\ation today should exhibit 

 types of sterility and should frequently fail to yield fruits and 

 seeds or even to produce flowers. 



A most obvious condition which enforces sterility of the 

 sweet potato is the non-blooming h.ibil. The plant is naturally 

 adapted to a warm climate and a long growing period and its 

 blooming is conditioned by this adaptation. Even its proper 

 vegetative growth is practically confined to those sections where 

 the average growing season is not less than 175 days and the 

 mean summer temperature is above 72° (Cioography of the 

 World's Agriculture, p. 97). But the i)Uint is able to thrive 

 vegetatively and to yield good crops of roots when the environ- 

 ment does not favor the formation of flowers. Throughout a 



