22 OUR NATIVE FERNS AND THEIR ALLIES. 



oosphere, now called the oospore, increases in size, and finally 



develops into a true fern. 



66. Pteridoid Phase. After the oo- 

 sphere has been fertilized it commences its 

 growth by the ordinary processes of cell 

 multiplication, and for a time remains with- 

 in the walls of the archegonium, which 

 continue to grow, until finally the interior 

 growth breaks through the walls, differen- 

 tiated into its first root and leaf. The young 

 fern draws its nourishment from the pro- 

 thallium for a time, but soon develops 

 root-hairs, which, extending into the soil, 

 maintain thereby an existence independent 

 of the prothallium. The latter growth hav- 

 FIG 16 Adiantum ing accomplished its work, withers away 



.hSr//;?un L g ten (Fig- '6). The first parts of the root, stem, 



seen from below; //, anc [ frond are very small and comparatively 



prothallium; b, first leaf; 



k, root-hairs^ of prothai- simple in structure, but those formed later 

 second w 'roots. firs ( After are successively larger, and not only bear 

 a closer resemblance to the mature form 

 of the species, but also develop increased complexity of struc- 

 ture. " The fern continues to gain strength, not by subsequent 

 increase of size of the embryonic structures, but by each succes- 

 sive part attaining a more considerable size and development 

 than the preceding ones, until at length a kind of stationary 

 condition is arrived at, in which the newly formed organs are 

 nearly similar to the preceding ones." 



67. The complete life-history of a fern illustrates a principle 

 common among the lower forms of animal life known as "alter- 

 nation of generations." Instead of the direct production of a 

 mature sexual plant, as among the higher forms of vegetation, 

 there is the production of a sexual growth resembling a lower 

 form of vegetation, which in turn is followed by the growth of 

 a mature plant producing its fruit without the assistance of sex- 

 ual organs. 



68. Recapitulation. To review the life-history of a fern 

 we find the following processes : 



