Germination. 



39 



Fig. -^7. Adianlum Ca- 

 pUlus-Venei^, L. Prothal- 

 linm and young fern seen 

 from below : pp, prothul- 

 lium ; b, first leaf ; h, root- 

 hairs of prothallium ; w, 

 fli"st and second roots. 

 (After Sachs). 



72. The Asexual ^Generation (Sporo- 

 phore.) — After the oosphere has been fertil- 

 ized it commences its growth by the ordi- 

 nary processes of cell multiplication and for 

 a time remains within the walls of the arche- 

 gonium, which continue to grow, until finally 

 the interior growth breaks through the walls, 

 differentiated into its first root and leaf. The 

 young fern draws its nourishment from the 

 prothallium for a time but soon develops 

 ro»(t-hairs which, extending into the soil, 

 maintain thereby an existence independent 

 of the prothallium. The latter growth having 

 accomplished its work withers away. (Fig. 

 27.) The first parts of the root, stem, and 

 frond are very small and comparatively sim- 

 ple in structure, but those formed later are 

 successively larger, and not only bear a 

 closer resemblance to the mature form of 

 the species, but also develop increased com- 

 plexity of structure. " The fern continues to gain strength, not 

 by subsequent increase of size of the embryonic structures, but by 

 each successive part attaining a more considerable size and de- 

 velopment than the preceding ones, until at length a kind of sta- 

 tionary condition is arrived at in which the newly formed organs 

 are nearly similar to the preceding ones." 



73. 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 ma- 

 ture 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 sexual organs. 



74. Recapitulation. — To review the life history of a fern we 

 find the following processes : 



A. Production of the spores asexually by the mature plant. 

 (Fructification.) 



B. Growth of the prothallium from the spore with or with- 

 out the development of a pro-embryo. (Sexual Generation. 

 Oophore). 



C. Production of sexual organs, archegonia (female) and 

 antheridia (male), on the under surface of the prothallium. 



