112 



certain conditions. Under the conditions at San Ramon 

 (and under tropical conditions in general), then, the char- 

 acteristically epiphytic groups have articulate stipes ; the 

 characteristically terrestrial ones, non-articulate stipes. 

 Among our Aspleniese a single genus exists which, when 

 mature, is always epiphytic in exposure ; namely, Steno^ 

 chlcsna. Its pinnae under these circumstances are articu- 

 late, but young plants near the ground are without ar- 

 ticulations. Our Pteridese include no epiphytes and no 

 plants with structural articulations. However, there are 

 some species of Adiantiim, notably A. opacum, the pin- 

 nules of which are deciduous in an emergency. The 

 Vittariese as a group are non-articulate epiphytes. To 

 endure this condition they have thick, rolling leaves with 

 very heavy epidermal walls and very few stomata. 



. The stems of the Polypodieceae are moderately modi- 

 fied in adaptation to a wide range of conditions — more 

 modified and more variously so than one might imagine 

 from text-book comparisons with Equisetum and Lyco- 

 podium. The most primitive form of fern stem is prob- 

 ably a short, erect one such -as is observed in Aspidium, 

 Diplazhim and Pteris. This may be subterranean, or 

 barely superficial, or in damp and darker places may 

 rise into the air, as is the case with most of the larger 

 Diplazia. On trunks deeply covered with vegetation 

 some small ferns have stems standing out radially with a 

 dense apical tuft of small fronds. A large number of 

 ferns lift their fronds above competition with their ter- 

 restrial neighbors by assuming the scandent habit. The 

 majority of these maintain their connection with the 

 ground, but are still, if we classify all ferns as either 

 terrestrial or epiphytic, rather to be regarded as belong- 

 ing to the latter class, because of their exposure. Nu- 

 merous other ferns keep to the ground or to their orig- 

 inal aerial support, but remove their leaves from com- 

 petition with one another by a creeping habit and by 

 bearing them at considerable intervals. Neither the geo- 



