296 FOUNDATIONS OF BOTANY 
minor vegetative development ( prothallia), while the spore- 
bearing plant is a leafy plant, even a tree in some ferns. 
The ferns in the strictest sense have sporangia derived 
from the epidermis (transformed hairs), while a few plants 
closely resembling them in general aspect (Botrychium, etc.) 
have sporangia formed in the tissue of the leaf. | 
In the next subdivision, the water-ferns (Fig. 215), there 
is little resemblance to the common ferns. The sporangia 
are in special receptacles at the basal portion of the plant. 
The spores are of two kinds, diwcious, one on germination, 
producing antheridia, the other archegonia. ‘This group 
includes two rooting forms, Marsilea (with leaves resem- 
bling a four-leaved clover) and P2lularia, bearing simple 
linear leaves, and two floating forms, Salvinia (Fig. 215) 
and Azolla. 
The remaining groups of fern-plants are the horse-tails 
and the club-mosses. The horse-tails have only one kind 
of spore and are peculiar chiefly in their vegetative aspect 
(Fig. 218), while the spore-bearing leaves, or sporophylls, 
are arranged in the form of a cone, as already shown. 
ote 
The club-mosses include some plants which, as their — 
name implies, have a superficial resemblance to a large 
moss, with the addition of a club-shaped stalked fruiting 
spike. These are the so-called “ground pines” and the 
running ground “evergreens ” used for Christmas festoons 
in New England. Technically the group is distinguished 
by the possession of firm-walled sporangia formed singly 
near the bases of the leaves. The ordinary club-mosses 
already referred to have but one kind of spore, while 
plants called Selayinella and Isoetes have two kinds of 
spores, in this respect resembling Marsilea. In many 
