Chapter XII 



THE VACUOLES 



Early data. Insuflficiency of methods. Theory of Hugo de Vries:- 

 Plant cells always contain, in their cytoplasm, watery inclusions 

 to which have been given the name vacuoles and which are the re- 

 gions of accumulation of numerous metabolic products. The 

 vacuoles contain a liquid called the vacuolar sap which holds in 

 solution very diverse crystalloid substances, such as mineral or 

 organic salts, mineral acids, sugars, and so forth. 



In cells in the process of differentiation, several vacuoles are 

 generally found but in most mature cells there is only one enormous 

 vacuole which occupies the greater part of the cell, the cytoplasm 

 forming about it only a thin parietal layer containing the nucleus 

 appressed to the cell wall. 



The splendid investigations of the Dutch botanist, Hugo DE 

 Vries, brought out the essential role of vacuoles in osmotic phe- 

 nomena of the cell and showed that a mature plant cell may be 

 likened to a small osmometer, composed of a semi-permeable ecto- 

 plasmic layer which lines the permeable cell wall on the inside, and 

 of a vacuole containing a solution of crystalloid substances. No 

 great modification is observed to take place if mature plant cells 

 are placed in water, for example, a staminate hair of Tradescantia 

 which has the advantage of being easily detached and of being 

 made up of large cells which lend themselves easily to observation. 

 However, it can often be observed that the vacuole dilates and 

 by its pressure causes a curvature of the cell wall. The vacuolar 

 sap, by virtue of the ciystalloid substances which it holds in so- 

 lution, has an osmotic pressure superior to that of pure water. 

 It is, therefore, hypertonic to water and so endosmosis takes place. 

 If the same cells are plunged into a solution hypertonic to the 

 vacuolar sap, these cells then show a remarkable phenomenon dis- 

 covered by DE Vries and called by him plasmolysis. There is pro- 

 duced an exosmosis which causes a contraction of the protoplasm. 

 The protoplasm becomes more and more detached from the cell 

 wall, and soon forms in the center of the cellular cavity a globular 

 mass separated from the cell wall, but remaining connected with 

 it by very fine cytoplasmic trabeculae which give evidence of ad- 

 herence of the cytoplasm to this wall. In elongated cells the pro- 

 toplasm as it contracts divides into several globular masses con- 

 nected like beads on a string by a thin cytoplasmic filament. This 

 phenomenon is caused primarily by the exit of water from the 

 vacuole, accompanied, of course, by a lack of imbibition on the 

 part of the protoplasm, but it is the contraction of the vacuole 

 which brings about the contraction of the protoplasm. Whenever 

 the cells on which the experiment is being carried out contain in 



