294 Introduction to Botany. 



substance secreted within the archegonium. After fertiliza- 

 tion, by coalescence with a sperm, the egg begins a series 

 of divisions resulting in the formation of a young fern plant, 

 whose leafy stem turns upward and seeks the light, and 

 whose root turns downward, penetrating the substratum. 

 The prothallium bears the sex organs and constitutes the 

 sexual generation. It contains chlorophyll, and is con- 

 nected with the substratum by absorbing hairs called 

 rhizoids, and is able to sustain an independent existence, in 

 some instances for several years. 



181. Comparison of Asexual and Sexual Generations. 

 We note that while the sexual generation is the more con- 

 spicuous in the mosses, it becomes subordinate in size and 

 differentiation in the ferns. In this respect the ferns may 

 be looked upon as intermediate between the mosses, 

 together with other lower plants, and the Spermatophytes or 

 seed-bearing plants. In the Spermatophytes, as in the 

 ferns, the asexual generation is the more highly developed, 

 the sexual generation being reduced to such an extent that 

 it requires good powers of the microscope to demonstrate 

 it ; while the asexual generation is composed of leaf, stem, 

 root, and all of the flower up to a certain stage of its 

 development. 



These facts can be summarized and compared by refer- 

 ence to the diagrams of Figs. 157, 158, 159. In these 

 figures the sporophyte is unshaded and the gametophyte 

 (after the germination of the spore) shaded. The fern will 

 be considered first, since in it the distinction between sporo- 

 phyte and gametophyte is most evident. 



When the fern spore (Fig. 157, d) germinates, a multi- 

 cellular body (A) is produced (prothallium) bearing sex 

 organs, namely, archegonia (a), containing each an egg 

 cell, and antheridia (), bearing sperms. Since the pro- 



