Nutrition of Multicellular Plants - 241 



APICAL CELL 



MIDRIB 



green parenchyma 

 colorless parenchyma 

 Avascular" cell 



Fig. 13-6. Diagram of longitudinal section of moss 

 plant. 



into the soil from the lower end of the stem, 

 are simple branched filaments of colorless 

 elongate cells, often twisted into stout root- 

 like strands. Like the true roots of higher 

 plants, the rhizoids serve for both absorption 

 and attachment; but structurally the rhizoids 

 are much simpler than the true roots (p. 246). 

 In most mosses, the blade of each leaf con- 

 sists of a single layer of rather thick-walled 

 cells, which contain numerous chloroplasts. 

 During photosynthesis — and this is the spe- 

 cial function of the leaf as a whole — each 

 individual cell absorbs carbon dioxide di- 

 rectly from the air. But water, which is 

 needed simultaneously in photosynthesis, is 

 obtained from the rhizoids, via the stem and 

 midrib of the leaf. The midrib, or central 

 axis of the leaf, is somewhat thicker than the 

 blade, due to the presence of several strands 

 of elongate cells placed end to end along 



the length of the leaf. This primitive vascu- 

 lar tissue serves not only to carry water and 

 salts out into the leaf, but also to take glucose 

 and other organic substances back from the 

 leaf to the other parts of the plant. 



The stem of a moss plant exhibits three 

 fairly distinct concentric zones: (1) an outer 

 /one made up of several layers of green cells; 

 (2) an intermediate zone of larger more 

 loosely packed colorless cells; and (3) an in- 

 nermost core of elongate slender cells (Fig. 

 13-6). Some photosynthesis occurs in the 

 green cells of the outer layers. However, 

 these cells have thick cutinized walls, and 

 their chief function is to waterproof the ex- 

 ternal surface of the stem — since very few 

 mosses possess a distinct epidermal covering. 

 The intermediate zone of thin-walled cells is 

 for the storage of reserve organic substances, 

 chiefly starch, although their loose arrange- 

 ment provides air spaces to facilitate the 

 respiration of the stem as a whole. The 

 primitive vascular tissue in the central core 

 of the moss stem serves for the distribution 

 of inorganic materials upward, and of or- 

 ganic products downward, through the plant. 

 In a few mosses the end walls of the vascular 

 cells — where they make contact with each 

 other end to end — are perforated, so that 

 protoplasmic continuity exists from cell to 

 cell, as in the sieve tubes of higher plants 

 (p. 242). Growth occurs by the multiplica- 

 tion of an apical cell at the top of each stem. 

 The leaves, and in a few species branches, 

 originate as buds from this growing point. 



Organs and Tissues of the Bryophytes. In 

 summary, the specialized organs and tissues 

 of higher land plants are foreshadowed only 

 feebly in the Bryophyta. Without true roots, 

 capable of absorbing water on a quantity 

 basis, the bryophytes have not developed an 

 extensive leaf surface, since such a surface 

 would expose the plant to an excessive loss 

 of water. Moreover, many of the Bryophyta 

 — that is, the liverworts — have not developed 

 even a primitive stem system. 



Likewise the tissues of bryophytes are only 

 partially differentiated. The epidermoid and 



