PTERIDOPHYTA. 231 



They are awl-shaped and have at the base a semi-amplexicaul 

 sheath, with a groove (/cwea), in which a sporangium is situated 

 (Fig. 238). The ligule is a foliar outgrowth from the upper edge 

 of the groove. The MACROSPORANGIA (each with a number of 

 macrospores), are situated on the outer leaves, the MICROSPORANGIA 

 (Fig. 238), on the inner ones. Between each cycle of fertile 

 leaves there are a number of imperfect or barren ones, as in the 

 case of the female plant of Cycas. The spores are liberated by the 

 decay of the sporangium. The two kinds of sporangia develope at 

 the commencement in the same way. The archesporium is, at 

 first, a hypodermal layer of cells which grow out in the direction 

 perpendicular to the surface of the leaf, and divide by a number of 

 walls parallel to this direction, forming a sporogenous mass of 

 cells. Some of the cell-rows of this sporogenous mass lose their 

 rich protoplasmic contents, and are arrested in their growth ; thus 

 incomplete divisional walls of sterile cells, " trabeculce" arise in the 

 sporangium, dividing it into a number of compartments one above 

 the other (Fig. 238 ). (The trabeculee, according to Groebel, play 

 the same part as the nutritive cells of the sporangium of Riella ; 

 the tapetal cells, as in the Ferns, are in a great measure dissolved 

 at a later period.) The sporogenous cell-rows, in the micro- 

 sporangia, give rise to a large number of spore-mother-cells, but in 

 the macrosporangia only one spore-mother-cell, with tapetum, is 

 developed from each fertile archesporial cell. 



The two native species, and several others, are aquatic plants, 

 the remaining species are land plants, or are amphibious. About 

 50 species. In temperate and tropical regions. FOSSIL species 

 in the Tertiary period. 



Order 2. Selaginellaceae. This order contains only one genus, 

 Selaginella. The STEM, in the majority of species, is dorsiventral, 

 long and slender, and apparently branches dichotomously, but in 

 reality monopodially, with well developed lateral shoots. The 

 LEAVES are small, round, or ovate, in the majority of species 

 arranged in whorls of two leaves each ; these whorls, however, 

 are not decussate, but are considerably inclined towards each 

 other, an arrangement by which four rows of leaves are produced, 

 each whorl having one large and one small leaf. The two leaves 

 in each whorl are of unequal size, the smaller one being placed 

 on the upper surface and the larger on the lower surface of the 

 stem (Fig. 240). Some species have spirally-arranged leaves, more 

 resembling the arrangement in the Lycopodiums. 



