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 
