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 
