52 Practical Plant Biology. 



vacuole not as an organ of the cell, but to view its expansions and 

 contractions as phenomena necessitated by the continuous ac- 

 cumulation of soluble products at one spot in a semi-permeable gel 

 like protoplasm. The osmotic pressure generated by the solutes 

 presses back the protoplasm and expands the cavity in which they 

 are, till the film separating the vacuole from the surrounding water 

 ruptures. The solution is then free to escape. This relieves the 

 internal pressure and the elasticity of the gel asserts itself and the 

 vacuole collapses, its walls being forced back by the gel recovering 

 from its distortion. As soon as the exit is closed, accumulation, 

 proceeding as before, causes a repetition of the process. It is evi- 

 dent that the phenomenon will only occur in those cells whose 

 protoplasm is not supported by a cell-wall. In others the tenacity 

 of the wall prevents the unlimited expansion and rupture of the 

 protoplasm. It will be noticed that among vegetable cells, it is 

 only the motile ones which do not possess a continuous cell-wall, 

 and hence only in motile cells would we expect to find contrac- 

 tile vacuoles. 



Our next subjects for study are some of the organisms con- 

 cerned with decay and putrefaction which we take as typifying 

 the vast assemblage of micro-organisms known as Bacteria, 



A very easy method of obtaining various examples of the 

 different forms of cells found in this group, is to gather a little 

 of any of the finer red sea-weeds from half-tide rocks, and to 

 keep them covered with salt water at room temperature for 

 two or three days. The water round the sea-weeds, which is 

 at first clear, becomes during the first day or so more and 

 more cloudy and turbid. When a drop of this cloudy fluid is 

 examined microscopically it will be found to contain considerable 

 numbers of protozoa, which may be seen swimming about in 

 a cloud of almost impalpable specks. The specks are bacteria. 

 When the high power is used these specks are just visible. Some 

 of them are motionless while some are constantly moving. Of 

 these latter some move with a steady motion, travelling for 

 long distances in a straight line, or on a large curve ; others 

 have an uncertain gait and move hither and thither with a 

 vacillating and flickering motion ; and again others dart on a 

 zig-zag path in a series of short dashes, pausing now and then 

 to indulge in a giddy whirl. The specks have no distinctive 

 colour and are apparent in the fluid owing to their high re- 

 fractive index. When the diaphragm of the microscope is 

 closed they appear like dark dust particles, when it is opened 



