320 HOW CROPS GBOW. 



only are self-fertile but cannot well be otherwise. Some 

 plants which carry these closed and inconspicuous subter- 

 ranean flowers depend upon tliem for reproduction by 

 seed, their large and showy aerial flowers being often bar- 

 ren, as in violets, or totally infertile ( Voandzeia. ) Flax 

 and turnips are self-fertilizing. 



Cross-Fertilization results from the contact of the 

 pollen of one flower with the ovules of another. In many 

 plants remarkable arrangements exist that hinder or 

 totally prevent self-fertilization and favor or ensure cross- 

 fertilization. 



In monwcious plants, as hazel or squash, flowers of one 

 sort yield pollen, others, different, contain the ovules ; 

 so that two distinct and more or less distant blossoms of 

 tlie same plant are necessary for seed-production. 



In the dmcious poplar and hops, the plant that pro- 

 duces pollen never carries ovules and that which bears the 

 latter is destitute of the former, so that two distinct 

 plants must co-opei'ate to form seeds. 



It often happens that the pollen of a flower cannot fer- 

 tilize the ovules of the same flower. This may be either 

 because the stigma is behind the pollen in development, 

 as in case of various species of geranium, or because the 

 stigma has passed its receptive period before the pollen is 

 mature, as in Sweet Vernal Grass {AntlioxantJium odo- 

 ratum). In both instances the ripened pollen may reach 

 stigmas that are ready in other flowers and fertilize their 

 ovules, insects being often the means of transportation. 



In a large number of flowers, whose pollen and stigmas 

 are simultaneously prepared, the position of the organs 

 is such that self-fertilization is difBcult or impossible. 

 The Iris,'Crocus, Pansy, Milk-weed {Asr.lepias), and many 

 Orchids, are of this class. The offices of insects in search 

 of nectar, or attracted by odOrs, are here indispensable. 

 The common red clover cannot produce seed without 

 insect aid, and the bumblebee customarily performs this 



