Lecture II. 



as the internal solutes cannot escape their particles become more 

 crowded and concentrated. At a certain stage the reduction 

 of volume of the vacuole brings the concentration of the solutes 

 in the vacuole to be the same, reckoned in dissolved particles, as 

 that of the external solution. This increase of concentration is 

 made apparent in the examples we have taken by the intensifica- 

 tion of the red colour of the vacuole ; for the dissolved pigment 

 like the other solutes has 

 been concentrated and 

 hence its colour deepens 

 proportionally. 



If a tissue whose cells 

 are thus in a state of plas- 

 molysis is transferred from 

 the salt solution into water, 

 very soon the osmotic pres- 

 sure of the solutes in the 

 vacuoles, no longer opposed 

 by an external osmotic pres- 

 sure, pushes the protoplasm 

 outwards and presses it 

 again against the cell-wall 

 and distends the latter. 



These observations show 

 that a cell which is abund- 

 antly supplied with water 

 will normally have its cell- 

 wall distended by osmotic 

 pressure. In this distended 

 state, which is called turgor, 

 it is rigid. One may com- 

 pare the rigidity of a football or pneumatic tyre the bladder or 

 the inner tube playing the part of the protoplasm, while the leather 

 or the cover is put into a state of tension like the cell-wall. 



The compressed air cannot penetrate the bladder, just in the 

 same way as the dissolved particles cannot pass through the 

 protoplasm, while the leather cover in one instance and the cell- 

 wall in the other takes up the pressure and prevents the inner 

 envelope extending indefinitely. The impermeability of the inner 

 envelope is necessary in each case and any injury to it which 

 would render it permeable will quickly destroy rigidity. A 

 temperature of about 70 C. changes protoplasm profoundly, and 

 among other changes taking place at this temperature, it will be 



FIG. 4. Cells of rhododendron petal, 

 x 103. c, cytoplasmic lining of cell ; 

 v, vacuole ; w, cell-wall. Below the 

 same cells are seen plasmolysed. 



