166 GEOLOGY. 



other plants, and enriching also barren soils by their de- 

 cay, so as to prepare them for higher forms of vegeta- 

 tion. These plants are wholly cellular in their composi- 

 tion, while all other plants, as grasses, trees, and shrubs, 

 have tubular vessels for the circulation of the sap. The 

 latter are called, therefore, vascular plants, and the for- 

 mer cellular. 



250. Acrogens. This division includes the mosses, the 

 ferns, the horsetails, and the ground-pine family. These 

 plants are pictured in Fig. 86, the tall tree-fern that 

 grows in tropical climates being represented as well as 

 the common fern, or brake, that is so familiar to us in 

 temperate climates. The plants of this class delight in 

 swamps and shady places. Their remains are found in 

 the peat of the present day, and in the coal deposits of 

 past ages. Their title comes from the fact that they in- 

 crease from the top alone, it being derived from two 

 Greek works, dkros, summit, and gennao. These, like 

 the amphigens, are of service in preparing soil for the 

 nourishment of higher orders of plants. They do a great 

 work in this respect, because they have a rapid growth, 

 and from year to year, by their decay, add to the solid, 

 nutritious material in the swamps and damp places where 

 they so luxuriantly nourish. Neither the amphigens nor 

 the acrogens afford support to animal life to any extent. 



251. Gymnogens. In this group, Fig. 87 (p. 168), we 

 have the cycads, or pineapple family, and the first pines, 

 or cone-bearing trees (eoniferse). They have this name 

 (gymnos, naked, and gennao) because their seeds are 

 naked instead of being inclosed in cases. They appear 

 in various kinds of soil, the dry as well as the damp. 

 From them, like the acrogens, peat and coal have been 

 largely formed. Our coal, both bituminous and anthra- 

 cite, was laid down in beds from the growth of both 

 acrogens and gymnogens far back in the past, as you 

 will see in another chapter. Their stiff, juiceless leaves, 

 scaly seeds, and hard berries afford but little nutriment 



