430 PLANT RESPONSE 



•001 mm. per minute, or one-fifth of the original rate. It was 

 only after about half an hour that the original rate of growth 

 was once more attained. 



I next applied warm water at the base, with the result 

 that the rate of growth was almost instantaneously enhanced 

 to "125 mm. per minute, or twenty-five times the normal! 

 That this effect was not due to the rise of temperature as 

 such, is shown by the fact that it was almost instantaneous, 

 and that, moreover, as will be shown in the next chapter, the 

 maximum rate of growth of Crmuni at the optimum tem- 

 perature is only about three or four times as great as the 

 normal. 



From these experiments we see that the energy applied 

 at the root is transmitted hydraulically to the growing region 

 by the ascent of sap, where a certain amount of work is per- 

 formed in causing an increase of turgidity, and thus producing 

 in the cells a state of tension. Growth is now caused not 

 simply by the presence of water, but rather by the energy 

 conveyed by that water. It is well to bear in mind, at this 

 point, that the mobility or plasticity of the responding grow- 

 ing region is also an important factor in the production of 

 growth ; for if the molecular mobility of the zone of growth 

 be in any way reduced, the transmitted pressure, which was 

 formerly effective, will now become ineffective to bring about 

 growth. 



(6) Effect of temperature on growth. — We have seen, 

 in studying the pulsatory movements of Dcsmodiuin, that the 

 rise of temperature, within certain moderate limits, increased 

 the rhythmic activity of the plant, as shown in the increased 

 frequency of pulsation. At a maximum temperature, again, 

 above 40° C. these movements almost disappeared, there 

 being now produced very rapid oscillations, of so small an 

 amplitude as to be almost incapable of detection. This will 

 be seen in fig. 175, where the normal pulsations, of a period 

 of 2'5' at 30° C, are seen reduced to a period of only 10" at 

 42° C. With this, the amplitude also is so far reduced as to 

 be visible only on very careful inspection. We meet with 



