298 



GENERAL BOTANY 



spores into two distinct kinds, which has been effected by some 

 of the higher living species and fossil species of the coal age. 

 These special features of their life histories indicate clearly 

 the important part played by the early ancestors of modern 

 ferns in the origin of the living seed plants. These important 

 features in the evolution of the plant body and reproduction 

 of the Pteridophyta will be more clearly understood after con- 

 sidering the life history of 

 type species selected from 

 the three great orders of liv- 

 ing Pteridophyta, namely, the 

 true ferns, the lycopods, and 

 the horsetails. 



FI LI GALES (FERNS) 



Habitat and habit. The 

 ferns are quite unlike the 

 mosses and liverworts in their 

 structure and mode of life, 

 since they are true land plants 

 with an organization similar 

 to that of the higher seed 

 plants. The great majority 

 of ferns inhabit the floor of 

 forests, where the light is 

 dimmed by overhanging foli- 

 age and the soil is moist and rich with decaying humus. A few 

 forms only are adapted to xerophytic conditions, and fewer 

 still are hydrophytes. Most ferns are therefore typical medium 

 land plants, or mesophytes, adapted to the conditions of light, 

 soil, and air found on the forest floor (Fig. 166). The leaves 

 are large -and divided to form the characteristic compound leaf 

 seen in most ferns. The leaf blades are thin, and the mesophyll 

 cells possess an abundance of chlorophyll, which is favorable to 

 the manufacture of starch and sugar in a dim and moist atmos- 

 phere ; the stem in most instances is an underground rhizome 



FIG. 167. Coiled tips (circinate vernation) 

 of a fern, Osmunda claytoniana 



