RELATION OF WATER TO THE PROTOPLASM 65 
described the strong salt solution failed to pass through 
the protoplasmic layer; the re-entry of the water into 
the vacuole showed that the protoplasm prevented the 
osmotic substances, originally present in the water which 
the cell contained, from escaping in the issuing stream. 
These substances must have been left behind, or there 
would have been no osmotically active material to draw 
the water back, when it was allowed to replace the salt 
solution outside the cell. 
‘That this behaviour is dependent on the vital activity 
of the protoplasm can be shown by repeating the experi- 
ment after killing the living substance by a short immer- 
sion of the cell mm alcohol. Then the process of osmosis 
goes on exactly as in the first experiment described. The 
salt solution penetrates into the vacuole as if only a cellulose 
septum were present, the dead protoplasm exerting no regu- 
lating influence. 
We must not conclude from this experiment that inor- 
ganic salts in all degrees of concentration are kept from 
entering the cell by the protoplasm. If extremely dilute 
solutions are employed, the living substance permits their 
passage together with a certain appropriate amount of 
water. Similarly, extremely dilute solutions of bodies 
found in the fluid of the vacuoles, the so-called cell-sap, 
can make their way out of the cells. The protoplasm 
exerts a definite regulating influence, however, upon both 
the entry and the escape of these different substances. We 
shall have occasion to discuss this more fully later. 
The regulated osmosis which is thus the mode of entry 
of water into a cell containing no vacuole, and which 
causes the growth or completion of the vacuole, after its 
first appearance, continues after its formation is finished. 
This can be studied most favourably in aggregations of 
cells, such as we find in the cortex of a stem or the loose 
mesophyll of a leaf, as m such cells there is a more evident 
renewal of the water of the vacuoles than in those of tissues 
which are surrounded by liquid. 
