Ch. XII] THE FERN PLANTS 493 



sues in the Pteridophytes, reach a degree of differentiation 

 far in advance of the Bryophytes, even rivaling if not equal- 

 ing the Spermatophytes. Second, they have developed 

 the shoot, and especially the leaves, in very high degree, 

 as manifest in the beautiful great fronds of the Ferns. Third, 

 they possess true roots, substantially like those of Sperma- 

 tophytes, and a great advance over the bryophytic rhizoids. 

 The main root, however, soon ceases to grow, and is replaced 

 by numerous adventitious rootlets. Fourth, while their 

 alternation of generations is morphologically identical with 

 that in Bryophytes, the relative prominence of the generations 

 is reversed, for the conspicuous green plant is now the 

 sporophyte, while the gametophyte is reduced to a much 

 smaller, and often insignificant, thalloid structure called the 

 prothallus. The chromosome numbers, however, keep 

 their former relations, and the reduction division still occurs 

 in the formation of the spores. Thus the half number pre- 

 vails through the gametophyte, with a doubling at fertili- 

 zation, and the full number occurs in all the cells of the sporo- 

 phyte. Fifth, the Pteridophytes in their higher forms 

 possess not a single kind of spore as in all lower groups 

 (homospory), but two kinds (heterospory), differing in size 

 and in the fact that one kind produces male, and the other 

 female, prothallia, or sexual plants. This arrangement is 

 carried farther in the next group, the Spermatophytes, 

 where the smaller male-producing spores become the pollen 

 grains, and the larger female-producing spores become the 

 embryo sacs of the ovules. The spore-cases are called 

 sporangia, and occur commonly in clusters, which are 

 often large and prominent. 



The reproduction in the Pteridophytes is essentially like 

 that of Bryophytes, even to the point that the sperm cells 

 are free-swimming, and require water for fertilization. This 

 fact makes it necessary that the sexual stage be kept where 

 such fertilization is possible, viz., close to ground which is 

 occasionally wet; and this factor greatly limits the distri- 



