33(5 PLANT STUDIES 



There are also ejiiplijilic forms (air plants) — that is, 

 those which pereh " upon other plants " hut derive no 

 nourisliment from them (Fig. i)5). ^J'his lialjit helongs 

 cliiefly to the warm and moist trollies, wlicrc the jilants 

 can absorb sufficient moisture from tlie air without send- 

 ing roots into the soil. In this way many of the trojiical 

 ferns are fouTrd growing upon living and dea.d trees and 

 other plants. In the temperate regions the chief epi- 

 phytes are Lieliens, Liverworts, and Mosses, the Ferns be- 

 ing cliietly found in moist woods and ravines (Fig. 302), 

 although a number grow in comparatively dry and exposed 

 situations, sometimes covering extensive areas, as the com- 

 mon brake {Pterin). 



The Filicales diifer from the other groups of Pterido- 

 phytes chiefly in liaving few large leaves, wliicli do cliloro- 

 pliyll work and hear sporangia. In a few of them there is a 

 differentiation of functions in foliage branches and sporo- 

 ])liyll brandies (Figs. 200, ;>00), l)nt even this is excep- 

 tional. Another distinction is that the stems a,re un- 

 branched. 



218. Origin of sporangia. — An important feature in tlie 

 Ferns is the origin of the sporangia. In some of tliem a. 

 sporangium is developed from a single epidermal cell of 

 tlie leaf, and is an entirely superficial and generally stalked 

 affair (Fig. 2!)(i, .7) ; in others the spurangimn in its devel- 

 opment involves several epidernnd and deeper cells of the 

 leaf, and is more or less of an inibediled alfair. In the first 

 ease the ferns are sai<l to he lfi)li)\jiiii-(iuij\(ili- : in tlu' sec- 

 oiul case they ai'e CKxiinnniiiialc. 



Another small but interesting group ol' !''cnis includes 

 the " Water-ferns," floating forms oi- sdmel imcs (in muddv 

 Hats. 'I'lie commdu Marsilin may he taken as a type ( i"'ig. 

 liOD). 'I'lu- slcndei' creeping stem sends down nninei-ous 

 roots into the mucky soil, and at in(cr\-als gi\'es rise to a 

 comparatively large loaf. Tliis leaf has a hnig ei'ect jietiole 

 and a blade of four spreading wedge-sluiped lea.flets like a 



