POLLINATION AND FERTILIZATION 



121 



the stigma as long as the flower remains undisturbed ; but at 

 a later period in the development of the organs, anthers and 

 stigmas may grow into contact with each other and self-polli- 

 nation be secured (Fig. 111). 



It is a remarkable fact that when a stigma is pollinated 

 with pollen from the same flower or from another flower of 

 the same plant, and also with pollen from another individual 

 of the same kind, generally only the latter 

 pollen takes effect in fertilizing the egg. In 

 other words, foreign pollen is prepotent over 

 pollen from the same individual. 1 



114. Self-pollination and cross-pollination. 

 The process of self-pollination is usually 

 rather simple, as may have been inferred 

 from Sect. 108. Not infrequently the be- 

 ginner in botany may be led to wonder 

 whether it would not be advantageous to 

 the plant world if all flowers were bisexual 

 and pollinated their own pistils. The matter 

 is not, however, quite as simple as it ap- 

 pears to be. The earliest seed plants were 

 doubtless remotely related to our evergreen 

 cone-bearing trees of to-day (such as pines, 

 spruces, and cedars), and these cone-bearers 

 have unisexual flowers (Figs. 251 and 262) 

 and are wind-pollinated. Bisexual flowers came later. It is likely 

 that, later still, plants with unisexual flowers have come into 

 existence by descent, with gradual modifications, from ancestors 

 which bore bisexual flowers. One proof of this is drawn from 

 the fact that there are many cases of flowers which are practi- 

 cally unisexual but show rudimentary pistils in the staminate 

 flowers and rudimentary stamens in the pistillate ones, as in 

 the common asparagus (Fig. 97). Occasionally the asparagus 

 has perfect stamens and pistils in the same flower. 



1 See Darwin, Cross and Self Fertilisation in the Vegetable Kingdom, 

 chap. x. D. Appleton and Company, New York. 



FIG. 111. Stamens 

 and pistils of round- 

 leaved mallow 



The flower has heen 

 open for a consider- 

 able time, and the 

 stigmas have curved 

 so as to come into 

 contact with the sta- 

 mens and insure self- 

 pollination. AfterH. 

 Miiller 



