MOSSES 239 



number of species of Mosses are small plants growing 

 on rocks and tree-trunks which can survive desiccation. 



The rhizoids are branched multicellular cell threads 

 which arise from the base of the stem and enter the 

 soil, from which they absorb water. The stem has a 

 central strand of long narrow thin-walled cells from 

 which the protoplasm has disappeared, and these 

 form a water channel from the absorbing region 

 (rhizoids) to the evaporating region (leaves). The 

 cells of the outer layers of the stem (cortex) have 

 thick brown walls. The leaves ordinarily consist of a 

 single layer of cells containing chloroplasts, but there 

 is often a midrib, several cells thick, consisting of 

 several cell-layers and possessing thin- walled water- 

 conducting cells like those of the centre of the stem. 

 The shoot grows by the division of a single apical cell 

 at its tip, as in the Liverworts and in Fucus. Each 

 segment cut off from this apical cell gives rise by 

 division to the tissue of a single leaf and to the segment 

 of the stem which bears it. 



The Moss plant is a stage further on than the 

 Liverwort in adaptation to land life. Though they 

 can absorb water through their leaves, many Mosses 

 have a regular water current from rhizoids to leaves. 

 The existence of a definite water-conducting tissue 

 corresponds with this localisation of absorption in one 

 part of the body (rhizoids) and of evaporation in another 

 (leaves), which is the mark of a terrestrial plant, and 

 which is absent in Fucus and almost absent in Pellia. 

 This feature is, as we shall see, carried to a much 

 higher level of development in the Vascular Plants. 



Sexual Organs and Sporogonium 0! Mosses. — The 

 sexual organs and gametes of the Mosses have a 

 general resemblance to those of Liverworts, though 



