168 - The Cell 





Fig. 9-8. One complete cell in a filament of the green alga, Spirogyra. The nu- 

 cleus, with a distinct darkly stained nucleolus, lies at the center, suspended in place 

 by delicate strands of cytoplasm. The chloroplast has the form of a spiral ribbon, 

 on which numerous small stained bodies, the pyrenoids, are discernible in this re- 

 touched photograph. (Copyright, General Biological Supply House, Inc.) 



such as Nitella (Fig. 13-2), are true multi- 

 cellular organisms. Algae differ from the 

 simple animals in that they all possess chlo- 

 rophyll, and most have relatively thick, rigid 

 cell walls. Usualh the chlorophyll is local- 

 ized within well-defined chloroplasts, of 

 which one or more may be present in each 

 cell. Sometimes the green color of the chloro- 

 phyll is masked by the presence of other pig- 

 ments, as in the case of brown and red algae. 

 Most algae differ from protozoa in having no 

 means of locomotion, although some species 

 possess one or more highly motile, whiplike 

 flagella (Fig. 9-7B). 



Algae, in contrast to most higher plants, 

 are essentially aquatic plants. A few dwell in 

 very moist places on the land, as for example, 

 Prqtococcus (Fig. 9-9), which may be found 

 on tree trunks, in the damp crevices of the 

 bark. But most algae grow under water. Clos- 

 terium, for example, is a unicellular form 

 very commonly found in fresh-water ponds 

 and lakes. 



The problem of absorption in a submerged 

 unicellular plant such as Closterium (Fig. 

 9-7A) is not very acute. All essential nu- 

 trients are right at hand, dissolved in the 

 surrounding medium; and all the food sub- 

 stances can enter the cell through the plasnra 

 membrane by diffusion and osmosis. Fresh 

 water, essentially, is a very hypotonic solu- 

 tion. The main solutes are inorganic salts — 

 which dissolve in rain water as it seeps 

 through the soil and collects in the pond — 

 together with the atmospheric gases, carbon 

 dioxide, and oxygen. These substances pass 

 into the cell by diffusion, although active 

 transport processes (p. 121) often intervene, 

 increasing or decreasing the intracellular 

 content of certain particular ions. 



Distribution. In unicellular plants such as 

 Closterium, the distribution of absorbed food 

 substances throughout the protoplasm is just 

 as simple as that in one-celled animals (p. 00). 

 Once within the plasma membrane, the dif- 

 ferent nutrients diffuse freely throughout the 



CELL WALL 



NUCLEUS 

 CHLOROPLAST 



CHLOROPLAST 



NUCLEUS 

 CYTOPLASM 



CELL WALL 



A 



Fig. 9-9. Profococcus, a unicellular green plant. A, temporary aggregates of cells, as they occur 

 naturally. B, single cell, more highly magnified, stained and sectioned. Chlorella (see p. 164) has a 

 structure similar, although not identical, to that of Profococcus. 



