152 Autonomy of Cell-Organs 



the starting point of new investigations. It has been, 

 without doubt, one of the most fruitful thoughts for the 

 development of our science. 



The further study of the cell-sap and the vacuoles, 

 suggested by this theory, has led in regard to the morpho- 

 logical aspect, which alone interests us here, to the proof 

 that the wall of the vacuoles is an essential, never wanting 

 part of the plant-protoplast. 51 The method which made 

 it possible always to demonstrate the presence of this wall 

 consisted in the treatment of the living cells with a 10% 

 solution of potassium nitrate, which has been stained with 

 eosin. Directly, or after a shorter or longer period, the 

 outer protoplasm dies in the reagent, while the wall of the 

 vacuoles remains living for a while. It is then visible 

 as a distended bubble, more or less completely separated 

 from the dead parts, and entirely preventing the penetra- 

 tion of the eosin. In colorless cells, therefore, the bubble 

 carries contents as clear as water, while the remaining 

 protoplasm is stained red or brown by the eosin. Fre- 

 quently the original vacuole separates into several smaller 

 ones; and not infrequently one can follow this process 

 directly under the microscope. 



The wall of the vacuoles is to be regarded as a special 

 organ of the protoplast, which regulates the secretion and 

 accumulation of the substances which are present in the 

 cell-sap in solution, and because of this function, it has 

 been given the name tonoplast. But frequently the sap- 

 spaces together with their walls are now designated as vac- 

 uoles. 



In living cells the tonoplasts are, as a rule, not visible, 

 because they consist of translucent vesicles of an extreme 



61 Vries, H. de. Plasmolytische Studien iiber die Wand der Vac- 

 uolen. Jahrb. Wiss. Bot. 16: 465. 1885. 



