SELECTIVE PERMEABILITY 77 



crushed, so that the contents of such 

 cells mingle, new reactions may result. 

 When a bitter almond is crushed, prussic 

 acid is at once evolved: in the normal 

 state the substances which produce this 

 reaction were kept apart by semiper- 

 meable membranes. Such membranes 

 may exist within a cell as well as at its 

 surface; if these internal membranes are 

 broken new reactions may start. It is 

 possible that this is what happens in the 

 sense of touch. The contact or pressure 

 may disrupt membranes within the cell 

 and so cause chemical reactions which 

 stimulate the nerves. The stimulus of 

 gravitation which causes the root to 

 grow downward may possibly come un- 

 der the same category. 



When a root which is growing down- 

 ward is placed in a horizontal position 

 there is a mechanical disturbance of the 

 protoplasm in each cell. A part of the 

 cell contents tends to sink downward 

 toward the bottom of the cell and in so 

 doing may rupture or displace some of 



