Nutrition of Multicellular Plants - 237 



it lacks chlorophyll. The rhizoid cell is de- 

 pendent on the other cells of the filament, 

 and sugars produced by photosynthesis in the 

 green cells are transferred to this colorless 

 cell by diffusion through the intervening cell 

 membranes. 



Among the most highly differentiated of 

 fresh-water algae is Nitella (Fig. 13-2). This 

 relatively large green alga may measure al- 

 most a foot in length. Nitella exhibits a 

 branching green "stem," which is attached 

 to colorless rhizoids at the lower end, and 

 which is surrounded above by whorls of 



green "leaves." Superficially, Nitella resem- 

 bles a higher plant; but microscopic study 

 reveals that the alga has a relatively simple 

 structure. The "stem" consists of a series of 

 long internodes, alternating with short nodes 

 (Fig. 13-2). Each node gives rise to a whorl 

 of slender "leaves," which otherwise are 

 similar to the main stem; and some nodes 

 give off branches like the main stem. One or 

 more of the lower nodes also sends off color- 

 less rhizoids, which attach the plant to the 

 soil or rock at the bottom of the pond. Ex- 

 cept for the nodal cells, all parts of Nitella 



Fig. 13-2. The water plant, Nitella, ex- 

 hibits some differentiation of its parts. A, 

 whole plant, about natural size. B, a 

 small portion, magnified. 



NODE 

 RHIZOIDS 



