130 Elementary Biology. 



thallus to the rapidly growing embryo, and corresponds to 

 that which was so largely developed in the embryo of the 

 moss, namely, the seta. The primary root appears first and 

 penetrates the ground, taking on its proper nutritive func- 

 tion ; subsequently the first leaf and the apex of the stem 

 emerge, and, bending round, pass to the upper surface of 

 the thallus. The thallus, or sexual generation, having now 

 fulfilled its function, withers away, whilst the young fern 

 develops gradually by division and differentiation of cells 

 into the organism already fully described above. 



It may prove by no means unprofitable if we pause at 

 this point and briefly review the phenomena of the life- 

 history of the fern, and draw attention to some of the more 

 important features viewed comparatively. 



In the first place, we have to observe the growing 

 importance of the asexual generation and the waning of 

 the sexual thallus. In the moss, it is true, we have this 

 condition reversed, the thallus being the important organism, 

 whilst the asexual plant was a mere parasite on it. In the 

 fern the asexual plant is by far the more important of the 

 two. 



Again, we have in the fern a perfect example of alterna- 

 tion of generations, unlike that of the fungus in being a 

 regular alternation, and differing from that of the moss in 

 that the two generations are for the greater part of their 

 lives totally independent of each other. 



Once more we have to observe the gradual simplifica- 

 tion and degeneration of the sexual organs. The ovarium 

 is having its canal cells reduced in number, and the venter 

 is becoming merely a hollow in the thallus, with which, 

 indeed, its tissue is continuous. The spermarium is also 

 simpler in structure, and gives rise to fewer sperms. 



It is also worth observing that the embryo fern is for 

 some time contained in the thallus which thus acts as a 

 nurse, and from which the embryo obtains nourishment by 

 means of a special organ, the so-called 'foot.' The embryo, 



