GROWTH OF PLANTS 139 



spondingly increase the yield. The osmotic pressure of the cell 

 sap of plants after irrigation diminishes appreciably. 



33. Influence of Chemical Stimulants on Growth. Toxic 

 Action of Pure Neutral Salts. Balanced Solutions. Chemical 

 Stimulation of Growth. — In considering the influence of various 

 external factors on growth, it is necessary to keep in mind the 

 following points. Growth is the result of complex chemical and 

 physical processes, proceeding in the cell under continuous 

 expenditure of energy, which is liberated through respiration. 

 Growth is closely connected with respiration and ceases as soon 

 as a plant is transferred to an oxygen-free medium. Other 

 external influences, with the exception of temperature, which 

 regulates the rate of all vital processes, and water, by which the 

 material for enlargement of cells is supplied, act upon growth 

 not directly but only indirectly, by producing certain deviations 

 of the processes on which growth depends. External factors in 

 nature, both chemical and physical, may act as growth stimuli. 

 Their characteristic peculiarity is the lack of relationship between 

 the quantity of the stimulant and the effect produced. Some- 

 times a feeble stimulus may produce a strong effect, and con- 

 versely, sometimes a strong stimulus is almost without influence 

 on growth. The effect depends largely on the character of the 

 stimulating agent and on the nature of its action on the normal 

 course of growth. The phenomena of irritability are often com- 

 pared to disturbances produced on a machine. Sometimes a 

 touch, which may control some important lever, can completely 

 change the movements of the machine, while a heavy shock 

 applied to other parts may produce no effect. Since it is not 

 known as yet what levers in the living machine, the protoplasm, 

 respond to the action of certain stimuli, it is not surprising that 

 these phenomena appear to be very complex and that in studying 

 them very contradictory experimental results have been secured. 



The effect of one of the physical stimulants, light, has been 

 discussed already (Art. 31). The influence of other stimuli, 

 chiefly of a chemical character, will now be considered. The 

 foremost place must be assigned to the action of cations and 

 anions. 



In discussing the colloidal state, it has already been noted that 

 the presence of electrolytes is absolutely necessary for the changes 

 in the state of colloids that continuously occur in the cell. Thus 



