CELL STRUCTUKE OF SPIROGYRA 77 



Draw a large figure of a cell showing the cell walls, plasma 

 membrane, chromatophore with pyrenoids, nucleus held in 

 place by the radiating protoplasmic strands, and the large 

 space in the interior of the cell free from protoplasm. 



B. Place a drop of iodine solution (Sec. 169) at the side of the 

 cover glass, and draw it under by means of a small piece of 

 filter paper applied against the opposite edge. 



1. Note the coloration, or staining, of the protoplasmic struc- 

 tures. The nucleus usually stands out sharply, and should 

 be drawn if it was not clearly seen in the unstained living 

 cell described in A. 



2. Draw a portion of the chromatophore showing a pyrenoid 

 under the highest magnification. There will probably 

 be found a circle of dark granules around the pyrenoid. 

 These are starch grains, manufactured by the chromato- 

 phore in the presence of sunlight, the process being called 

 photosynthesis. 



C. Plasmohjsis. The shrinking of the protoplast away from the 

 cell wall when the cell is bathed in a denser solution, as that of 

 salt (described in Sec. 6Q, D), is Cd^W^diplasmolysis. Plasmoly sis 

 is accomplished by the withdrawal of water from the interior 

 of the protoplast through the permeable plasma membrane 

 and cell wall when there is a denser solution outside of the 

 cell. Such a movement of water through a permeable mem- 

 brane is due to osmosis {Principles, Sec. 48). The plasma 

 membrane of the protoplast is normally held against the 

 cell wall in the living cell by pressure from within, and that 

 condition is called cell turgor. The fluid within the proto- 

 plast IS termed cell sap and is contained in cavities called 

 vacuoles. The cell sap of Spirogyra is in one large vacuole 

 occupying the central region of the cell, in which the nucleus 

 is swung like a hammock by radiating strands of proto- 

 plasm. If the facts and principles illustrated by plasmolysis 

 in Spirogyra are not clear, repeat the experiment outlined in 

 Sec. 5&. D. 



