THE 



225 



can be tested by treating some plants exposed to light and some 

 kept in darkness with iodine solution. The plants exposed to 

 light will give the blue reaction for starch in the starch sheath ; 

 the plants which have been in darkness for some hours will fail 

 to give this test, showing that the stored starch has been digested 

 and used. In this case, if the plants are reexposed to light 

 they will be found to form starch again in 

 a very few minutes. 



Physiology. Spirogyra, like Protococcus, 

 is not materially different from higher 

 plants as regards the essential processes of 

 nutrition, cell division, and reproduction. 

 The cell sap contains an abundance of 

 dissolved inorganic food materials, which 

 the cells readily absorb, like a root-hair cell, 

 by osmosis. Carbon dioxide and oxygen 

 are likewise easily obtained from the water 

 in which the plants float, or it may be 

 obtained directly, from the air when large 

 masses of these algye rise to the surface of 

 ponds or lakes, buoyed up by the gas bub- 

 bles liberated during photosynthesis. 



Digestion of the excess starch formed 

 and temporarily stored in the pyrenoids 

 during the day is known to take place at 

 night, as in the leaf cells of higher plants. 

 Since it is also known that the cells of 



Spirogyra usually divide and elongate during the night, it is 

 probable that this is a period of active assimilation and respira- 

 tion in the individual cells of the filaments. 



Through the above processes Spirogyra cells grow and attain 

 the maximum size fixed for each species of Spirogyra plant. 

 When this maximum size is reached, each cell may divide and 

 form two new cells by a division wall which is laid down by the 

 cytoplasm across the middle of the cell. This cell division takes 

 place in the following manner: The new dividing wall first 

 forms a thickening, ringlike ridge of cellulose on the old cell 



-Cell wall 

 Pijrenoid 

 Nucleus 

 Cytoplasm 

 Vacuole 



cell 

 icall 



FiG^llS. Cell struc- 

 ture and cell division in 

 Spirogyra 



The cells are shown as 

 they appear in a longi- 

 tudinal section. Observe 

 the relation of pyrenoids, 

 cytoplasm, and nucleus 



