CHAPTER XXXII. 



COMPARISON OF FERNS AND THEIR RELATIVES, 



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

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

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

 leaves. While these phmts are not so lari^e in size as some of the ferns, 

 still we see that there lias been a great advance in tlie 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 micrnsporangia with 

 microspores, and some bear macrosporangia with only niacro-j-xjres. 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 

 sporophvlls, one kind of spore, tlie 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 ^]K)re is developing. Thus, while 

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

 it is really depen<lent 'in it for support or n<.Hirishint.'nt. 



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

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

 which i^ the fern plant, lead^ an independent existence Jroui the pnithalliuni. 

 and fonn'^ root, ^tcm, and lea\'rs. The spnre^ are borne in sporangia on 

 the leaves. The pnttlialliuiu also leads an indepen<lent existence, though in 

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



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