INDUCED RHYTHM IN TRANSPIRATION. 73 



In studying the behavior of the plants of experiments 75 and 79 it must be 

 borne in mind that the conditions to which the pieces were exposed were quite 

 normal for the time of year, that is, the temperature was steadily rising and 

 the relative humidity, which is always low, steadily decreasing. In all cases 

 an early morning low maximum is developed. Following the reduction of 

 rate after the low maximum has been attained, there is a steady rise to the 

 normal daily maximum. 



Experiments 176 to 179, inclusive. July 18, 19, 1905. 



Exposed to normally increasing illumination, but all showed a low maxi- 

 mum at 4 a. m. (see fig. 13 and accompanying data). 



CONCLUSIONS. 



As I have shown under another caption (pp. 59 and 62), the evidence is 

 lacking that the behavior of the ocotillo, as regards the fluctuations in the 

 transpiration rate in the early morning when in total darkness, is due to 

 stomatal activity with corresponding fluctuations. Similar changes of rate 

 under normal illumination would seem, alone, to indicate this. Even should 

 a small increase be shown to occur in the dimensions of stomata, it would 

 still remain to show that this caused the fluctuation in transpiration rate, 

 though the important question of a possible induced rhythm in stomatal 

 movement would be answered. 



It is possible that a slow increase in the dimensions of the stomata, such 

 as probably occur during prolonged darkness, may account for the frequently 

 seen slow increase in transpiration rate after midnight, but this will not ex- 

 plain, I believe, the production of a maximum followed by a reduction of rate. 

 That such a change, though more pronounced, occurs in plants exposed to 

 normal conditions would also be an objection to the explanation based on 

 stomatal activity. 



The constant occurrence of the early morning maximum in pieces kept in 

 total darkness is very remarkable, and would seem to be properly interpreted 

 as the expression of an induced rhythm. It would also appear, however, that 

 the physiological changes which lead to the maximum commence long before 

 the hour of normal daily stimulation at dawn. This rhythm appears to be 

 quite independent of the rhythm in water absorption by the roots (Eberdt) 

 and in bleeding (Hof meister and others) . 



The best we can say at present is that the plant behaves qualitatively 

 the same in total darkness as under normal conditions until an hour (usually 

 near 6 a. m.) when, the normal stimulation being absent, the plant relapses 

 to a low condition of activity characteristic of its darkness condition. Un- 

 doubtedly the early morning maxima observed by Curtis are the same thing. 

 For the lack of a real explanation we must refer the behavior to the category 

 of induced rhythm. 



