233 - Multicellular Plants 



are syncytial, with several nuclei enclosed 

 within each wall; and except lor the rhizoids, 

 all parts of Nitella contain chloroplasts. 



These structural differentiations in Nitella 

 indicate a corresponding functional speciali- 

 zation, or division of labor, among the differ- 

 ent parts of the plant. The rhizoids serve tor 

 attachment, and being incapable of photo- 

 synthesis, they depend upon the green parts 

 for organic sustenance. The "leaves" are in a 

 particularly favorable position for absorbing 

 sunlight, and perform the major part of 

 photosynthesis. The internodes. because of 

 their elongate form and active protoplasmic 

 streaming, specialize in the distribution of 

 synthesized organic substances to the other 

 parts of the plant. At the end of each stem 

 and branch there is an apical cell, which 

 continually divides to form the cells of a 

 new node; and the nodal cells retain their 

 capacity for dividing and differentiating into 

 the various specialized parts of the Nitella. 

 New "stem" and "leaves" originate as buds 

 from the nodes in a manner thai is char- 

 acteristic of the growth of higher plants. 

 However, "stems" and "leaves" of Nitella are 

 syncytial, rather than multicellular struc- 

 tures. 



Some larger marine algae (seaweeds) pos- 

 sess multicellular rhizoids, or "holdfasts," 

 which are rootlike organs (Fig. 13-3). Rut 

 rhizoids have a far simpler structure than 

 true roots, being composed mainly of elon- 

 gate cells twisted into stout strands, and the 



rhizoids serve only for attachment. A chief 

 function of a true root is to absorb water and 

 dissolved substances, but in the algae this 

 function is performed by all the cells indi- 

 vidually. Many large seaweeds also develop 

 structures that externally resemble the true 

 stems and leaves of higher plants, although 

 internally these organs display only a small 

 degree of cellular differentiation. 



THE LAND AND ENVIRONMENT 



To survive on land, plants have had to re- 

 solve a serious dilemma. In the soil, water, 

 salts, carbon dioxide, and oxygen are avail- 

 able — but there is no light. And above the 

 soil — that is, in the air — light is available, 

 but the water supply is inadequate and in- 

 organic salts are lacking. Therefore a truly 

 successful land plant must possess parts that 

 extend down into the soil and parts that 

 reach up into the air. Accordingly, the condi- 

 tions of the terrestrial habitat have deter- 

 mined the development of roots, stems, and 

 leaves, which are the main nutritive organs 

 of all well-adapted land plants. Typically. 

 roots extend down into the soil to anchor 

 the plant in an erect position and to absorb 

 enough water and dissolved materials to 

 meet the needs of the whole plant. The 

 leaves are the specialized organs of photo- 

 synthesis, which pass on extra glucose and 

 other organic compounds to the rest of the 

 plant; and the stem supports the leaves and 



Fig. 13-3. Holdfasts of a seaweed {Microcystis) from the California coast. 



