136 EVOLUTION OF PLANTS 



sporangiates, in spite of their numbers, might all be 

 included in a single order, Filices, were it not for 

 a small number of the heterosporous forms, i.e., those 

 having two kinds of spores, which are, however, evi- 

 dently related to the Filices. A study of the different 

 orders of the Eusporangiatse indicates that we have to 

 do with remnants of once much larger groups, of which 

 most of the members have become extinct. The much 

 greater homogeneity of the Leptosporangiatse, as well 

 as their numbers, indicate on the other hand a special- 

 ized and presumably more modern type of vegetation, 

 and this is borne out by a study of their distribution. 



None of the Eusporangiatse ever occur in any great 

 numbers together, although some of them are cosmo- 

 politan. One order, the Marattiacese, are strictly tropi- 

 cal plants, and usually occur as isolated individuals or 

 in small groups. Among the Leptosporangiates, on 

 the other hand, the plants are often gregarious, and 

 form conspicuous features of the vegetation. The com- 

 mon brake, Pteris aquilina, and in the tropics, species of 

 Gleichenia, form tangled thickets and cover extensive 

 tracts almost to the exclusion of other vegetation. If 

 we analyze the fern-flora of those tropical regions 

 where ferns form an important feature of the vege- 

 tation, the disproportion in numbers between the eu- 

 sporangiate and leptosporangiate species is even greater 

 than in temperate regions. Thus in Jamaica, which is 

 exceptionally rich in ferns, out of about five hundred 

 species described from the island, less than a dozen are 

 eusporangiate, and of these none are common enough 

 to make any impression upon the general character of 

 the vegetation, although an occasional gigantic Marattia 



