240 - Multicellular Plants 



Also the upper surface is equipped with 

 clearly defined openings, the pores, each 

 flanked by specialized pore cells (Fig. 13-5). 

 The pore cells control the size of the numer- 

 ous openings, tending to close them when 

 the quantity of escaping water vapor be- 

 comes excessive. The pores lead into air 

 spaces, inside the thallus, and these spaces 

 permit carbon dioxide to diffuse from the 

 outside air to the delicate green cells, or 

 chlorenchyma, which border the air spaces. 

 The remaining thickness of the thallus is 

 composed mainly of larger cells with fewer 

 chloroplasts. These cells serve chiefly as a 

 storage tissue, in which extra quantities of 

 photosynthesized glucose are deposited in the 

 form of starch grains. In addition to the 

 storage tissue, the cells in the central parts 

 of the thallus, especially along the "midribs" 

 (Fig. 13-5), are unusually long and display 

 very active cyclosis. This primitive vascular 

 tissue accelerates the distribution of sub- 

 stances from one part of the thallus to an- 

 other. 



The rhizoids (Fig. 13-5) are colorless elon- 

 gate cells with delicate walls, adapted to the 



absorption of water and mineral salts. The 

 rhizoids occur in clusters and each cluster is 

 protected by one or more scales, which are 

 multicellular extensions of the lower epi- 

 dermoid layer. In dry weather, the scales 

 tend to curl around the rhizoids, protecting 

 them from desiccation. 



Growth of the thallus is by the prolifera- 

 tion of apical cells at the end of each lobe. 

 Frequently a growing point becomes divided 

 into two masses of embryonic tissue, each of 

 which generates a new lobe; and this method 

 of growth accounts for the branching habit 

 of the thallus. 



The Mosses. The general form of the moss, 

 with its small erect leaf-surrounded stem, was 

 described previously (p. 208). Like the liver- 

 worts, typical mosses are only semiterrestrial. 

 They grow in densely crowded clumps, pro- 

 tecting each other from desiccation; and ex- 

 cept for a few highly developed species, the 

 mosses are restricted to damp and shady 

 localities. 



The internal (microscopic) structure of a 

 moss plant is shown diagrammatically in Fig- 

 ure 13-6. The rhizoids, which extend down 



CUT SURFACE 

 EPIDERMOID CELLS, 



PORES 



CHLORENCHYMA 

 EPIDERMOID TISSUE 

 STORAGE CELLS 

 LOBE OF THALLUS 



PORE 



PORE CELLS 



AIR SPACE 



RHIZOIDS 



SCALES 

 RHIZOIDS 



Fig. 13-5. A liverwort {Mar- 

 chantia). This plant shows 

 some very primitive adapta- 

 tions that enable it to live on 

 land, but only in places where 

 water is abundant (see text). 



