REPRODUCTION 453 
to the visiting insect, for when it touches the stamens of a 
short-styled form, it covers with pollen the part of its 
body which will come into contact with the stigma of the 
next long-styled flower it alights upon. Another portion of 
its body will be dusted with the pollen from the latter, which 
will be suitably placed to be deposited upon the stigma of 
the next short-styled form it may visit. The best seeds are 
produced when each stigma is supplied with pollen from 
stamens occupying a corresponding position to itself. This 
method of cross-pollination is not thoroughly effective, as 
the insect after a short time will be carrying pollen from 
stamens of both lengths, having visited several flowers of 
both kinds. The size of the pollen-grains in each case is, 
however, correlated with the features of the corresponding 
stigmatic surface, which helps to secure the most advan- 
tageous result. 
This arrangement is termed heterostylism or dimorphism, 
of which, however, it is only one form. Lythrum Salicaria 
is trimorphic, bearing two sets of stamens of different 
lengths, and a style which differs from both. There are 
three modes of arrangement of these organs, and, as in the 
Primrose, the most serviceable pollination is that which 
takes place when pollen from a stamen of a particular 
length is applied to a stigma in a corresponding position. 
Other arrangements are physiological rather than struc- 
tural. Of these the strangest is what is called prepotency. 
When a stigma of a flower exhibiting this property is 
pollinated by pollen from its own stamens, and at the same 
time by pollen taken from another flower, the latter is 
always the originator of the gamete by which fertilisation 
is effected. Some flowers show self-sterility—that is, their 
oospheres are incapable of being fertilised by generative 
nuclei developed from their own pollen; in some few cases 
their own pollen acts as a poison to them. 
Though cross-pollination is generally most advantageous 
it is not universal. Self-pollination occurs in many plants ; 
in some, indeed, special means have been developed to secure 
