PRACTICAL EXERCISES 19 



iodine solution, and observe the following results of that 

 treatment : 



1. The cell-wall will not be appreciably altered or stained. 



2. The film of protoplasm (primordial utricle) will have 



stained a pale yellow or brown, and may often be 

 seen to have separated partially or completely from 

 the cell- wall, from which it may now be readily 

 distinguished. 



3. The chromatophores will have assumed a dusky colour, 



while the pyrenoids will be a dark purple. 



4. The nucleus will have stained a deep yellow or brown, 



and inclosed within it one or sometimes two xmcleoli 

 may be recognized by their deeper colour, and high 

 refractive power. 



b. Irrigate a fresh specimen with 2^ per cent, solution of 

 common salt, and watch the result, which will take some 

 minutes to appear. The protoplasm will gradually separate 

 from the cell-wall, and while the latter retains its form and 

 position, the protoplasm will contract, and rounding itself off, 

 ultimately appear as a more or less irregularly oval or spherical 

 body. 



c. Mount a fresh specimen in water, and irrigate with 

 glycerine; the cells will collapse, owing to the sudden ab- 

 straction of water of the cell-sap. 



d. Mount a fresh preparation in water : irrigate with potash 

 solution, and observe that the protoplasmic contents swell 

 and lose their definite outline, and the whole becomes more 

 transparent. 



II. The above example of a simple Alga shows cells aggre- 

 gated in a linear series, constituting a filament : the next 

 specimen illustrates the more complicated arrangement of cells 

 in two dimensions of space, i.e. as a flat plate of tissue. 

 Mount a small prothallus of a Fern (or the thin lateral part of 

 a large one) in water, and examine it under a low power : such 

 Fern prothalli are commonly to be found on the damp surface 

 of soil or flower-pots in ferneries where the air is constantly 

 damp : or they may be readily grown by sowing Fern-spores 

 on moist soil. 



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