318 GENETICS IN RELATION TO AGRICULTURE 
(d) Dicecious 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 moncecious 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 Aa 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 
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