GROWTH 307 



moisture supply, however, may have certain unfavorable conse- 

 quences. The development of protective tissues, the cuticle and 

 cork, is weaker in moist air and hence the plant is more exposed to 

 the attacks of parasitic fungi, whose spores germinate readily 

 in a moist medium. Moreover, in many plants air with a high 

 moisture content retards the ripening of fruits. Excessive moist- 

 ure in the soil makes it swampy (Art. 46), which is detrimental to 

 most plants. Most mesophytes and. especially xerophytes develop 

 better in moderately moist habitats. 



92. Effects of Chemical Stimulants on 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 com- 

 plex chemical and physical processes, proceeding in the cell under 

 continuous expenditure of energy which is liberated through res- 

 piration. Thus 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, do not act upon 

 growth directly, but only indirectly, by producing certain devia- 

 tions of the processes on which growth depends. These altera- 

 tions are due to the plasticity of the factors involved. Their 

 characteristic peculiarity is the lack of relationship between the 

 amount of the acting stimulant and the effect produced. Some- 

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

 times, quite the contrary, a strong irritation is almost without 

 influence on growth. The effect depends largely on the nature 

 of the stimulating agent, and on the degree and the direction of 

 deflection of the normal course of growth. The phenomena of 

 irritability are often compared to disturbances produced on a 

 working machine. Sometimes a feeble touch, but one that may 

 cause to turn some important lever, may 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 very contradictory experimental 

 results have been secured. 



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

 cussed already (Art. 90). The influence of other stimuli, chiefly of 



