CHAPTER IX 



CHANGES IN PERMEABILITY OF PLANT CELLS 



That the permeability of a cell may change has long been 

 known. Nageli (1855) observed that when plant cells died 

 chlorophyll and other substances diffused out of them. Pfeffer 

 ("Pflanzenphysiologie") explained the movements of plants as 

 due to changes in permeability. It had been shown by Pfeffer and 

 de Vries that the osmotic pressure of many plant cells is very 

 high — when the cells are placed in water, that they do not burst 

 is due to the strength of the cell wall. If water is accessible the 

 plants are firm and elastic as a rubber tube when it is filled with 

 water under pressure. If the water is removed from them by 

 evaporation or by immersion in a concentrated salt solution they 

 wilt and contract. The same is true if the permeability of 

 the cells is increased, as by killing the plant by immersion in 

 hot water or in some toxic solution or gas. But changes in 

 permeability may occur without death of the plant. Such 

 changes follow some external change which we call a stimulus. 

 Plants which respond in this way to stimuli are called sensitive 

 plants. The leaves of Mimosa pudica, and many Leguminoceae 

 and Oxalaceae close after being touched. The stamen hairs of 

 Cynara (French artichoke), Centaurea, Berberis and Helianthe- 

 mum shorten after an insect walks over them. 



Pfeffer observed the shortening of these stamen hairs under 

 the microscope. A fluid exuded from the cells into the inter- 

 cellular spaces, due to increased permeability of the cells to the 

 osmotic substances which held the water in them. The cell walls 

 had been stretched by an osmotic pressure of about three atmos- 

 pheres. When this was released the cells decreased in volume 

 and the hair consequently decreased in length. The cells in such 

 cases are not dead because they gradually absorb water and swell 

 (regain their turgor) and the hair lengthens. The osmotic sub- 

 stances that were lost when the permeability increased cannot 



