THE VARIETY OF PLANT LIFE 



95 



the two phyla of the leafy spore plants have the simplest types of 

 plant bodies. 



The phylum Bryophyta consists of some 17,000 species of 

 small plants which can well be called the amphibians of the plant 

 world, since they prefer to live in semi-aquatic habitats. Theirs 

 are the least complex organs of any land plants, and in their life 

 history (see p. 55) occurs an alternation of a green independent 

 gametophyte with a parasitic colorless sporophyte. At least 12,- 

 000 of the species are included in the group of Bryophytes known 



Fig. 65. — Mosses are predominantly small plants; common examples are 

 crane's-bill (A), Mnium (B), and rock moss (C). 



as Mosses (fig. 65), which are predominantly small plants a few 

 inches in height and with the habit of growing in closely packed 

 masses. The "leaves" which appear on the erect stems are not 

 structurally true leaves, being only a few cells in thickness and 

 made up of a homogeneous tissue of unspecialized cells. However, 

 superficially they resemble leaves. Mosses lack roots, absorbing 

 their water and minerals as we have seen through rhizoids. The 

 only genus of any economic importance is peat moss, whose 

 deposits are used as fuel. 



The other group of Bryophytes is known as the Liverworts. 

 They are generally prostrate and creeping plants with no leaf- 

 like organs as in the mosses (fig. 66). Many liverworts consist of 

 forking thalli with little greater tissue specialization than that of 



