296 FOUNDATIONS OF BOTANY 



minor vegetative development (prothalUa), 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, dioecious, one on germination, 

 producing antheridia, the other archegonia. This group 

 includes two rooting forms, Marsilea (with leaves resem- 

 bling a four-leaved clover) and Pilularia, 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. 213), while the spore-bearing leaves, or sporophylls, 

 are arranged in the form of a cone, as already shown. 



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 Selaginella and Isoetes have two kinds of 

 spores, in this respect resembling Marsilea. In many 



