318 GENETICS IN RELATION TO AGRICULTURE 



(d) Dioecious plants. Flowers of different sexes on different plants, 

 thus insuring cross-fertilization. Examples: hemp, hops, asparagus and 

 date palm. 



Another class having hermaphrodite and uni-sexual flowers on the 

 same plant is termed polygamous. The sunflower might be classified 

 here, because its marginal ray flowers are pistillate only. Certain species 

 of Composite have the marginal flowers pistillate, through complete sup- 

 pression of the anthers as in the sunflower itself, and the disk flowers are 

 hermaphrodite, but the pistil always aborts, so that in effect they are 

 really monoecious plants. In some cases, however, they are known to be 

 completely self-sterile, so that cross-fertilization must always take place 

 in seed formation. 



The above classification requires numerous qualifications. For ex- 

 ample, it has been our purpose to list under Class A a those plants which 

 are so generally self-fertilized that it is not necessary to protect them to 

 insure self-fertilization, but there are some species and varieties among 

 them which sometimes exhibit a significant amount of cross-fertilization. 

 The cultivated varieties of wheat are very rarely cross-fertilized, but the 

 wild wheat of Palestine has a floral mechanism especially designed for 

 cross-fertilization. Some varieties of rice, also, are cross-fertilized often 

 enough in mixed plantings to make it impossible to assume self-fertili- 

 zation in a given selection. In peas and beans, perhaps, the proportion of 

 crossing is greater than in the cereals mentioned above, and in some cases 

 it is absolutely necessary to protect them from insect activities. Thus Pearl 

 and Surface in breeding investigations with Yellow Eye beans found it 

 necessary to enclose selected plants in large muslin cages in order to 

 exclude bumble bees, which were found to be effective enough agents 

 of cross-pollination in open fields to disturb results greatly. On the 

 other hand, however, Pearl and Surface in extensive investigations in 

 oat breeding report not a single case of natural crossing. Also Rimpau, 

 who carried on extensive investigations with nineteen varieties of oats 

 over a period of six years, observed only five cases of spontaneous hy- 

 bridization. Furthermore in most of the commonly cultivated varieties 

 of wheat, barley, and rice natural crossing is so rare a phenomenon as 

 to be worthy of special note in any observed case. We recall also 

 Johannsen's pure line investigations with Princess beans which would 

 have been impossible had natural crossing occurred among them in any 

 significant amount. 



Among plants having hermaphrodite flowers which are usually self- 

 fertilized there is also vast difference in the relative proportions of self- 

 and cross-fertilization. In cotton, Balls has found it necessary to allow 

 for about 5 per cent, of natural crossing. In tobacco self-fertilization 

 is the rule, but it is not sufficiently assured to obviate the necessity for 



