336 PLANT STUDIES 



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

 those which perch " upon other plants " but derive no 

 nourishment from them (Fig. 95). This habit belongs 

 chiefly to the warm and moist tropics, where the plants 

 can absorb sufficient moisture from the air without send- 

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

 ferns are found growing upon living and dead trees and 

 other plants. In the temperate regions the chief epi- 

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

 ing chiefly 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 (Pteris). 



The Filicales differ from the other groups of Pterido- 

 phytes chiefly in having few large leaves, which do chloro- 

 phyll work and bear sporangia. In a few of them there is a 

 differentiation of functions in foliage branches and sporo- 

 phyll branches (Figs. 299, 300), but even this is excep- 

 tional. Another distinction is that the stems are un- 

 branched. 



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

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

 sporangium is developed from a single epidermal cell of 

 the leaf, and is an entirely superficial and generally stalked 

 affair (Fig. 296, 5) ; in others the sporangium in its devel- 

 opment involves several epidermal and deeper cells of the 

 leaf, and is more or less of an imbedded affair. In the first 

 case the ferns are said to be leptosporangiate ; in the sec- 

 ond case they are eusporangiate. 



Another small but interesting group of Ferns includes 

 the " Water-ferns," floating forms or sometimes on muddy 

 flats. The common Marsilia may be taken as a type (Fig. 

 303). The slender creeping stem sends down numerous 

 roots into the mucky soil, and at intervals gives rise to a 

 comparatively large leaf. This leaf has a long erect petiole 

 and a blade of four spreading wedge-shaped leaflets like a 



