CHAPTER XIII. THE PTERIDOPHYTA. 357 



The female prothallium partly protudes as a hemispherical 

 mass from the ruptured wall of the macrospore, but most of it 

 remains concealed within the coats of the spore. 



The fructification consists of a modified leaf-blade of pod- 

 like appearance, which contains several sori, each enclosed in 

 an indusium. In each sorus are sporangia which contain macro- 

 spores, and others which contain microspores. See Fig. 564. 

 The Pilularias are closely related to Marsilia, and are classed in 

 the same group. All the members of the order are aquatics. 



Class III. — The Lycopodine^e, or Club Moss Group. 



The plants of this class are, for the most part, small or 

 moderate sized perennial herbs, sometimes with stems erect and 

 rooting at the base, but more commonly creeping, with ascending 

 or erect branches, which, in the majority of cases, originate dich- 

 otomously, though in some instances monopodially. With few 

 exceptions, the stems and branches are thickly clothed with 

 leaves. The latter, in the simplicity of their structure, are in 

 strong contrast to those of the Ferns, being mostly of small size, 

 without petioles or stipules, usually provided with but a single 

 nerve, which constitutes a mid-vein, and the blade is never 

 branching or compound. 



The roots differ from those of most other plants, in being 

 dichotomously branching. The sporangia are, in the majority 

 of cases, borne on the leaves, but, in some instances, as in Psi- 

 lotum and Selaginella, on the stem. The spore-bearing leaves 

 may be of the ordinary form, or thiey may be somewhat modified 

 in structure and crowded together, forming cones or spikes at 

 the ends of some of the branches. Some members of the group 

 bear but one kind of spores, others produce both macrospores 

 and microspores. 



There are three orders, the Lycopodiacea, the Psilotcuea and 

 the Ligulata. 



Order A. The Lycopodiaceae. All the existing forms of 

 the group are isosporous, but the Lepidodendrons of the Coal 

 Age, which must be regarded as belonging here, were hetero- 

 sporous. The prothallia, which have been studied in only two 

 species, are well-developed, subterranean, tuber-like bodies, 

 which bear both antheridia and archegonia. 



