62 GENERAL BIOLOGY 



osmotic pressure, if one only replace the water beneath the 

 cover glass with some denser liquid, such as dilute glycerine, 

 or 5% salt solution. This outer film will then be seen to 

 shrink away from the cell wall, and if the shrinkage con- 

 tinues, to collapse altogether; but if replaced quickly in 

 pure water, it is soon restored to its original condition; 

 clearly the larger part of the cell is occupied with the watery 

 cell sap, easily withdrawn or replaced. 



Thus the main features of structure may be seen in the 

 living cell. But the relations of some of the more delicate 

 parts may be made more clear by the two following experi- 

 ments: If a drop of iodine solution be placed upon the 

 fibres upon the slide, it will stain the protoplasm yellowish 

 brown, making the peripheral parts of it more apparent. 

 It will also stain the minute starch granules that lie about 

 the edges of the pyrenoids dark blue or blackish. 



If a few fresh green filaments be placed in 

 strong alcohol, the chlorophyl will be dissolved 

 out by the alcohol (more rapidly with the aid 

 of heat) and the protoplasmic matrix in 

 which the chlorophyl is held will be apparent. 

 Nitella. In order to get a large enough 

 single mass of pure protoplasm to see without 

 lenses and to handle, it is necessary to find 

 cells much larger than the ordinary ones, that 

 shall contain it. The common stonewort, 

 Nitella, is an alga with some very large 

 (multinucleate) cells, from which the proto- 

 plasmic content is easily removable, and may 

 well be used for a first direct observation on 

 protoplasm. Nitella grows upon submerged 

 limestone rocks in permanent water. It is one 

 f the most highly organized of the algae. It is 

 fntemod.*' . attached at its base, bears branches arranged 



