THE ALGJE 223 



The free-swimming forms of algse, like Chlamydomonas, are 

 undoubtedly more primitive types of plants than nonmotile 

 forms like Protococcus, which have apparently become adapted 

 to a dry habitat with the loss of motility. 



SPIROGYRA 



Despite its specialized character, Spirogyra has been selected 

 as the first type of a filamentary alga to be studied, on account 

 of its availability and the ease with which its cellular structure 

 and reproductive processes can be demonstrated. It forms a 

 considerable part of the green scum frequently seen on the 

 surfaces of stagnant pools in the summer and is easily recog- 

 nized by its slippery feeling, due to the gelatinization of the 

 outer layer of the cell walls of its constituent cells. 



Structure. Each cell of the filament is a cylinder with a 

 cellulose wall and a thin cytoplasmic sac surrounding a large 

 central vacuole. The large nucleus and its nucleolus are sus- 

 pended in the center of each cell by strands of cytoplasm ex- 

 tending from the cytoplasm surrounding the nucleus to the 

 cytoplasmic sac (Fig.117, A). The nucleus is thus bathed by 

 the nutrient cell sap of the water vacuole. 



Unlike the cells of higher plants, there is no difference 

 between the cells composing the filaments of Spirogyra, and no 

 differentiation into tissues or organs. Each cell, therefore, like 

 the independent cells of Protococcus, is capable of performing 

 all of the functions of life. Absorption, photosynthesis, diges- 

 tion, assimilation, and respiration are thus carried on by each 

 cell independent of all the others. It follows that if Spiro- 

 gyra filaments are broken up by the waves so as to separate a 

 cell or a group of cells from its fellows, such cells or cell groups 

 will grow and produce new plants by repeated division and 

 elongation. Since each cell performs all of its own functions, we 

 shall expect to find its different cell structures highly complex. 

 This is particularly true of the chloroplasts, which, in addition 

 to the function of photosynthesis, serve as a temporary store- 

 house for the starch formed by it during the hours of daylight. 



