GROWTH 215 



by the following simple experiment. If the increase in 

 the volume of cells and, therefore, of whole organs 

 depends on the supply of water attracted by the solution 

 in the vacuole, by surrounding the cell or the organ 

 instead with a solution that will draw the water out of 

 the vacuole we shall cause a decrease in its volume. 

 Let us actually observe under a microscope a living 

 cell in a solution of sugar or salt more concentrated 

 than the cell-sap. We shall notice that the volume 

 of the cell will decrease (fig. 63, i and 2), and when 

 the wall consisting of solid matter cannot reduce itself 

 any longer the protoplasm will loosen itself from the 

 cell-wall, and owing to its surface tension will follow 

 the further decrease of the vacuole (fig. 63, 2), and in the 

 end shrink into a regular little ball (fig. 63, 3). This 

 shrinking of the protoplasmic bladder is termed for the 

 sake of brevity plasmolysis. Evidently we may con- 

 clude that the fundamental mechanism of growth is 

 the converse of that of plasmolysis.^ Even if we have 

 no microscope at hand we can easily prove the truth 

 of what has been said. Let us take the succulent 

 stem of any herbaceous plant, measure its length 

 exactly, and place it in a solution of common kitchen salt 

 containing five per cent, of salt. After a short time let 

 us measure its length again. We shall notice that the 

 stem has considerably shortened, which means that here 

 we have a phenomenon the reverse of growth. Suppose 

 we transfer the stem back into water. It will regain its 

 former size, and go on living and growing. Therefore 

 this experiment gives us an opportunity for verifying 

 our explanation of the mechanism of growth, without 

 injuring the stem in any way. 



If the increased pressure of the sap on the cell-wall be 



* We have indicated this symbolically with arrows in fig. 63. As has 

 been already said, the wall, consisting of solid matter, is unable to 

 contract as completely as the primordial utricle. C means cellulose; 

 Pr. u.- — primordial utricle ; Fac— vacuole. 



