THE EVOLUTION OF THE PLANT BODY 



33 



but among the mosses, near relatives of the liverworts, leaf-like 

 organs appear. 



A Primitive Leafy Land Plant 



Many mosses prefer to grow in damp and shaded situations 

 similar to those chosen by liverworts, but they are able to live also 

 in sunnier and drier habitats, since they are structurally better 

 fitted for terrestrial life. Mosses have stems on which grow leaf- 

 like organs which contain most of the 

 chloroplast-bearing cells. The basal end of 

 the stem develops rhizoids similar to those of 

 Marchantia. Still lacking, however, are the 

 true roots and the vascular stem tissues 

 found in the fern* and seed plant groups. 

 This deficiency has predestined mosses to 

 being a small and insignificant portion of 

 our vegetation. 



The HAIRY-GAP MOSS of fields and road- 

 sides (fig. 13) is typical of the group. Each 

 individual plant is rarely more than a few 

 inches in height, and grows in close associ- 

 ation with numerous other individuals to 

 form a compact mossy turf. There are three 

 divisions to the moss body. In contact with 

 the earth is a mass of filamentous rhizoids, 

 threads of colorless cells whose function is 

 the same as in the liverworts. The rather 

 frail stem is made up of closely packed cells 

 lacking chloroplasts, and showing only slight 

 specialization for support or conduction. The leaves are hardly 

 true leaves as we understand them in terms of those found on 

 trees and flowering plants. They are simple outgrowths from the 

 stem, made up of chloroplast-bearing cells. The maintenance 

 activities of a hairy-cap moss plant are practically the same 

 as those of Marchantia. Water passes from the rhizoids by inter- 

 cellular diffusion through the stem to the leaves, or directly from 

 the environment into the rather unprotected leaf cells. Carbon 

 dioxide and other gases pass into the leaf cells from the atmos- 



FiG. 13.— Hairy 

 cap moss has a frail 

 stem clothed with 

 leaf-like outgrowths, 

 and possessing absorb- 

 ing rhizoids. 



