94 James Small on 



the under side than in the upper side of the root. The effect of 

 the passage of an electiic current through the normal plant cell 

 is an increase of what is called permeability ; this results in a 

 decrease of turgor and a decrease in the rate of growth. The 

 ultimate result in the root is that both the under and the upper 

 sides grow at a slower rate than before, but the under side, being 

 affected more, grows slower than the upper side, with the natural 

 result that the root curves doAvnwards. 



The relationships between permeability, turgor and growth 

 require to be clearly understood. The sap in the cavity within 

 the cytoplasm of a mature plant cell acts like the gas in a 

 balloon, and the cytoplasmic membrane acts like the skin or 

 fabric of a balloon. If the balloon be well blown up it is more 

 or less rigid or "turgid"; similarly with the cell. The sap or 

 gas may be considered as always efficient in itself, but if the 

 membrane be leaky ( = permeable) then no amount of "blowing up" 

 will make the cell or the balloon turgid. When the membrane 

 in the cell is so altered that the molecules in solution in the sap 

 can pass through ( := permeate or leak) more easily than before, 

 the cell loses a certain amount of its turgidity or turgor. The cell 

 grows because its wall is always being stretched by the internal 

 pressure ( = osmotic pressure) of the sap. If the cytoplasmic 

 membrane be leaky, the sap exerts less pressure, therefore each 

 cell grows at a slower rate and the tissue, which is built up of 

 many such cells, elongates less rapidly ; it grows more slowly. 



How THE Stem grows Up. 

 The events which result in the turning-up of a stem, when 

 it is i)laced horizontally, are very similar in kind to those 

 previously described for the root. In the stem, however, the 

 protein particles carry an electro-negative charge, because the rest 

 of the cytoplasm is relatively alkaline. This difference has a very 

 important effect on the direction of growth. The particles cream 

 upwards as before, when the stem is placed horizontally, but since 

 they arc negatively charged the current which is developed flows 



