REPRODUCTION IN PLANTS. 123 



Stigma may be seated directly upon the ovary, or may be 

 separated from it by a longer or shorter stalk, ^vhich is 

 termed the "style" {b). 



The male and female organs of reproduction are usually 

 present in the same flower, when the plant is "monoecious;" 

 but at other times one individual produces the male flowers 

 and another individual produces female flowers, when the 

 species is " dioecious." Even in bisexual flowers, however, 

 there is reason to believe that there are natural arrange- 

 ments whereby perpetual self-fertilisation is prevented, and 

 the influence of another individual is at intervals secured. 



In ordinary cases amongst Angiosperms, the process by 

 which the ovule is impregnated may be described as fol- 

 lows : — The anthers, when ripe, burst, and shed their con- 

 tained pollen upon the moist stigmatic surface of the pistil. 

 The viscid secretion of the stigma seems to act in such a 

 manner upon the pollen-grains that their inner lining is 

 protruded in the form of delicate microscopic tubes — the 

 " pollen-tubes." These insinuate their extremities into the 

 loose tissue of the stigma, and, gradually elongating, make 

 their way into the ovary ; the distance traversed in this 

 way varying with the distance between the stigma and ovule, 

 and being enormously great in long-styled plants. During 

 this process, changes have been going on in the ovule, in 

 consequence of which impregnation is possible. The most 

 important of these consists in the enlargement of the so- 

 called " embryo-sac," which truly corresponds with the 

 ovum of animals, and the formation in its interior of from 

 one to three or more vesicular bodies, which are known as 

 the " embryonal vesicles," and which seem to correspond 

 with the germinal vesicle of the ovum of animals. When 

 the pollen-tube reaches the embryo-sac, its further growth 

 seems to be generally arrested, and it is only in rare cases 

 that the pollen-tube perforates the embryo-sac,* if, indeed, 



* Recent researches demonstrating the possibility of cells making 

 their way through unbroken surfaces, as has been incontestably proved 



