86 PLANT LIFE ON THE FARM. 



one flower exerts its influence, not upon the germ of its 

 own flower, but upon that of another, perhaps situated 

 on some other plant. Cross-fertilization is often neces- 

 sitated by the circumstance that while the pollen of any 

 particular flower may be ripe, the stigma and the germ- 

 cell of the same flower may not be ripe at the same time, 

 or vice versa, and in such case the co-operation of some 

 other flower is needed. 



Transport of Pollen. It becomes, therefore, needful 

 to ascertain in what manner the pollen is carried from 

 one flower to another. This is effected in various ways 

 sometimes the mere splitting of the anther with some 

 degree of force suffices to scatter the pollen, at other 

 times the currents of air suffice to waft it from one flower 

 to another, while at other times insects of various kinds 

 convey the pollen from one flower and deposit it on the 

 stigma of another. 



The adaptations of floral structure to insect-agency in 

 fertilization, as also the contrivances for excluding unde- 

 sirable visitors, are most varied and most remarkable, 

 but they can only be mentioned here. As a rule, it may 

 be stated that flowers endowed with bright colors, irregu- 

 lar construction (like that of the pea or bean), or rich 

 perfume are fertilized by insect agency. The insects are 

 attracted by the bright colors, the perfumes, and the 

 sweet secretions of the flower. On alighting, they are 

 often compelled by the peculiar construction and mech- 

 anism of the flower to enter or leave it in such a way 

 that they must in the one case brush out the pollen, and 

 in the other deposit it on the stigma. It will be noted 

 how this process of cross -fertilization favors that process 

 of variation to which allusion has previously been made. 



The farmer wishing to keep his stock of seed turnips 

 pure, knows how difficult it is to do this, not only be- 

 cause cross-fertilization is the rule in the particular 



