DISTRIBUTION OF FERNS. 33 



The Tree-Ferns are all found in tropical or sub- 

 tropical countries. 



It is difficult to ascertain exactly how the ferns 

 of the various countries of the globe compare in 

 quantity with the other plants of the same dis- 

 tricts ; but it is safe to say that the proportion of 

 ferns in quantity is larger as the proportion in 

 number of species increases. The following gives 

 some idea of the ratio of the number of species of 

 ferns to that of flowering plants : 



Jamaica . . . i fern to 8 flowering plants. 

 New Guinea " 4 " 

 Tropical America . " 35 " " 

 Portugal . "105 " 

 Greece ... " 227 " 

 U.S., east of the Mis- 

 sissippi . "46 



The great majority of ferns are perennial. Only 

 a few are annual ; Gymnogramme leptophylla, G. 

 chcerophylla, and Ceratopteris thalictroides being 

 examples. The latter is also aquatic, almost 

 the only fern that is so, for it grows in shallow 

 water, with the sterile fronds floating on the sur- 

 face of the stream. 



Many ferns, instead of growing in the earth 

 in the usual way, force their roots deeply into 

 the crevices of rocks ; as, Notholana (PI. 2), Pel- 

 l<za (PI. 4), &c. Still others, as Vittaria and 

 Nephrolepis, are epiphytic, growing upon trees, 



