THE MICROSCOPE AND CELL-STRUCTURE. 65 



get in, or any water flows over the upper side of the cover- 

 glass, start all over again. 



(a) Using the low power objective, note the numerous colourless 

 cells, more or less egg-shaped, either isolated or connected in groups ; 

 some may have been injured by teasing, but most of them will be 

 alive. Find a place where the cells are not too crowded, and put 

 on the high objective. Note, hi a cell, the thin smooth cell-wall, 

 lined inside by a film of fine-grained protoplasm, and the large cell- 

 cavity (vacuole). 



(b) Put a drop of salt solution on the slide, close to one edge of 

 the cover-glass, but do not let it flow over the upper side of the 

 cover (if it does, make a fresh preparation), then push a bit of 

 blotting-paper along to the opposite edge of the cover, so as to 

 irrigate the cells with the salt solution as the blotting-paper soaks 

 up the water. Watch the cells ; the protoplasm film shrinks from 

 the cell-wall as the cell is plasmolysed, but the wall itself shows no 

 apparent change. 



(c) Mount a fresh bit of pulp in the same way in a drop of water 

 and irrigate with glycerine (or mount at once in glycerine) ; the cell 

 is strongly plasmolysed, as with the salt solution, and the cell-wall 

 also shrinks and becomes folded as the cell collapses. 



(d) Treat another preparation with iodine solution ; the wall is 

 not stained (or only stained faintly yellow or brown), but the proto- 

 plasm becomes dark-brown and a small rounded or oval body 

 (nucleus) becomes prominent by staining more deeply than the rest 

 of the protoplasm. 



(e) Mount another preparation in Schulze's ohlor-zinc-iodine, and 

 let it stand for a few minutes ; the protoplasm stains brown, as 

 with iodine solution, but the cell-wall is stained violet (reaction of 

 cellulose). 



98. Spirogyra. Mount in water and examine with the 

 microscope a small quantity of Spirogyra, a green filamentous 

 freshwater Alga, commonly found in floating masses, in 

 summer, on ponds and slow streams. 1 



(a) The threads feel slimy, owing to a coating of mucilage on the 

 surface. Notice the unbranched filaments, each consisting of a row 

 of cylindrical cells, separated by cross-walls at intervals, and each con- 

 taining one or more green spiral bands. 



1 Spiroyyra is supplied by Mr. Bolton, 25, Balsall Heath Road, 

 Birmingham, at almost any time, in shilling tubes. Any other material 

 for practical work can be bought from Mr. Bolton, or from the British 

 Botanical Association, Holgate Nurseries, York. 



S. B. 5 



