118 



BOTANY. 



small, and often scarcely protrudes beyond the spore, and may 

 be reduced to a single archegonium or antheridium (Fig. 71, 

 .B, C) with only one or two cells representing the vegetative 

 cells of the prothallium (v). The water ferns are all aquatic 

 or semi-aquatic plants, few in number and scarce or local in 



their distribution. The com- 

 monest are those of the 

 genus Marsilia (Fig. 71, A), 

 looking like a four-leaved 

 clover. Others (Salvinia, 

 Azolla) are floating forms 

 (Fig. 71, D). 



Of the true ferns there are 

 a number of families distin- 

 guished mainly by the posi- 

 tion of the sporangia, as well 

 as by some differences in 

 their structure. Of our com- 

 mon ferns, those differing 

 most widely from the types 

 are the flowering ferns (Os- 

 munda), shown in Figure 70, 

 (7, D. In these the sporan- 



FIG. 71. A, Marsilia, one of the Rhizo- 

 carpesR (after Underwood), sp. the 

 "fruits" containing the sporangia. 

 B, a small spore of Pilularia, with 

 the ripe antheridium protruding, 

 x 180. C, male prothallium removed 

 from the spore, x 180. D, Azolla 

 (after Sprague), x 1. 



gia are large and the ring (?*) 

 rudimentary. The leaflets 

 bearing the sporangia are 

 more or less contracted and 

 covered completely with the 

 sporangia, sometimes all the leaflets of the spore-bearing leaf 

 being thus changed, sometimes only a few of them, as in the 

 species figured. 



Our other common ferns have the sporangia in groups (sori, 

 sing, soms) on the backs of the leaves. These sori are of 

 different shape in different genera, and are usually protected 

 by a delicate membranous covering (indusium). Illustra- 



