238 PRACTICAL COURSE IN BOTANY 
Charles Darwin, 
proved by experi- 
ment that the seeds 
produced by polli- 
nating a dimorphous 
flower with its own 
pollen, or with pol- 
len from a flower of 
similar form, are of 
Fics. 348-350. — Three forms of loosestrife (Lyih- Very inferior quality 
rum salicaria). to those produced 
by impregnating a long-styled flower with pollen from a 
short-styled one, and vice versa. 
271. ‘Nature abhors self-fertilization.’’— These are the 
three principal methods by which nature provides against 
self-fertilization. Other cases occur in which the relative 
position of the two organs is such that self-pollination is 
difficult, or impossible, as in the iris and bear’s grass; or the 
pollen may be incapable of acting on the stigma of the flower 
that produced it. This aversion to self-fertilization is so 
great that many flowers, even when capable of it, will give 
preference to the pollen of another plant of the same 
kind, if dusted with both. From his observations on the 
behavior of plants in reference to this function, Charles Dar- 
win drew the conclusion that “Nature abhors perpetual 
self-fertilization.”’ 
272. Cleistogamic flowers.— Apparent exceptions to this 
rule are the hidden flowers found on certain plants which 
seem to have been constructed with a special view to self- 
fertilization. They are called cleistogamic, or closed, because 
they never open, but are fertilized in the bud; and those of 
the fringed polygala do not even rise above ground at all. 
Flowers of this kind can be found on several species of 
violet, concealed under the leaves, close to the ground; and 
the flowers of the peanut, found in the same situation, while 
they open slightly, are close-fertilized and practically cleisto- 
