238 



PRACTICAL COURSE IN BOTANY 



348 349 349 



Figs. 348-350. — Three forms of loosestrife {Lyth- 

 rum salicaria). 



Charles Darwin, 

 proved by experi- 

 ment that the seeds 

 produced by polH- 

 nating a dimorphous 

 flower with its own 

 pollen, or with pol- 

 len from a flower of 

 similar form, are of 

 very inferior quality 

 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- 



