How Plants Perpetuate Their Kinds 



281 



spores are provided with coats of such thickness and hardness as 

 to make them immune against every natural condition of dry- 

 ness and heat for weeks, months or years together; which fact, in 

 conjunction with their power of floating with the dust in the air, 

 explains their ubiquitous penetration into 

 all kinds of strange places. It is thus 

 that Bacteria and Yeasts, for example, 

 are enabled to spread so widely as they 

 do. 



Such is asexual reproduction, which 

 never involves fertilization, and has no 

 relation whatsoever to sex. In sharp 

 contrast stands sexual reproduction, in- Fig. 95.— The spore case of a 



1 • ,1 .. ,• ,1 1 p .-T Mold, discharging its asexual 



volvmg the cooperation, through fertiliza- spores; highly magnified. 

 tion, of two parents which are usually, f^'"""^ t '"^h-copied pic- 



' ^ -^ ' ture by Brefeld.) 



though not always, of different sexes. 



We may best begin our studj^ of fertilization with the more famil- 

 iar plants, where it occurs in the flower, which is a structure 

 specially adapted thereto. In a typical flower, as everyone 

 knows, the outer green protective calyx and the inner colored 

 showy corolla together enclose the stamens and the pistils (fig- 

 ure 96). The stamen consists of a slender stalk crowned by a 

 chamber, the anther, containing a fine yellow dust, the grains of 

 pollen, inside of which develop the male cells of the plant. The 

 pistil consists of a rounded chamber, the ovary, extending upwards 

 into a lengthened stalk, the style, ending in a roughened swelling, 

 the stigma, and containing one or more ovules in which develop 

 the female cells of the plant. If, further, a critical examination is 

 made of a typical ovule, by aid of longitudinal sections and 

 microscope (figure 97), it is found to enclose inside of some coats 

 a definite cavity, the embryo sac, within which in turn is consider- 

 able protoplasm and several cells, including a larger one close to 

 the end where an opening (the micropyle) is left through the coats. 

 This larger cell is the female generative cell, the exact equi\'alent 



