CHAPTER XXXII. 



COMPARISON OF FERNS AND THEIR RELATIVES. 



596. Comparison of selaginella and isoetes with the ferns. On compar- 

 ing selaginella and isoetes with the ferns, we see that the sporophyte is, as 

 in the ferns, the prominent part of the plant. It possesses root, stem, and 

 leaves. While these plants are not so large in size as some of the ferns, 

 still we see that there has been a great advance in the sporophyte of selagi- 

 nella and isoetes upon what exists in the ferns. There is a division of labor 

 between the sporophylls, in which some of them bear microsporangia with 

 microspores, and some, bear macrosporangia with only macrospores. In the 

 ferns and horsetails there is only one kind of sporophyll, sporangium, and 

 spore in a species. By this division of labor, or differentiation, between the 

 sporophylls, one kind of spore, the microspore, is compelled to form a male 

 prothallium, while the other kind of spore, the macrospore, is compelled to 

 form a female prothallium. This represents a progression of the sporophyte 

 of a very important nature. 



597. On comparing the gametophyte of selaginella and isoetes with that 

 of the ferns, we see that there has been a still farther retrogression in size 

 from that which we found in the independent and large gametophyte of the 

 liverworts and mosses. In the ferns, while it is reduced, it still forms 

 rhizoids, and leads an independent life, absorbing its own nutrient materials, 

 and assimilating carbon. In selaginella and isoetes the gametophyte does 

 not escape from the spore, nor does it form absorbing organs, nor develop 

 assimilative tissue. The reduced prothallium develops at the expense of 

 food stored by the sporophyte while the spore is developing. Thus, while 

 the gametophyte is separate from the sporophyte in selaginella and isoetes, 

 it is really dependent on it for support or nourishment. 



598. The important general characters possessed by the ferns and their 

 so-called allies, as we have found, are as follows: The spore-bearing part, 

 which is the fern plant, leads an independent existence from the prothallium. 

 and forms root, stem, and leaves. The spores are borne in sporangia on 

 the leaves. The prothallium also leads an independent existence, though in 

 isoetes and selaginella it has become almost entirely dependent on the sporo- 



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