406 LECTURE XVI. 



in the shoots of plants growing in the open air, it can only be 

 observed during the summer but not in the winter. It does 

 not appear, however, that variations of temperature contri- 

 bute largely to the production of the daily periodicity, for 

 Kraus found that only very considerable variations of tem- 

 perature, far greater than those which usually occur in any 

 one period of twenty-four hours during the summer months, 

 materially affect the intensity of the tensions. For instance, a 

 very marked increase in the tensions is produced when a shoot 

 which has been kept at a temperature of 7-8 C. is exposed 

 to a temperature of 1 5 2OC. So long as the temperature 

 varies within certain normal limits, say between ioC., and 

 3OC., the variations do not appear to affect the daily period. 



Millardet has observed a similar daily periodicity of the 

 tensions in Mimosa pudica, the Sensitive Plant. His observa- 

 tions confirm the statements of Kraus with regard to the 

 relation of the daily period to variations in the external condi- 

 tions in all respects except with regard to the influence of 

 temperature. Millardet found, namely, that a rise of tem- 

 perature from 1 8 to 32 C. materially increased the tension, 

 and that a fall from 30 to 19 C. materially diminished it. 



Kraus ascribes the daily periodicity of the tensions to 

 variations in the turgidity of the parenchymatous tissues, par- 

 ticularly of the pith. When a shoot is insufficiently supplied 

 with water, it is the pith which is most affected, as is shewn 

 by the fact that it becomes shorter, under these circumstances, 

 than any of the other tissues. This explanation is confirmed 

 by results obtained in his subsequent investigations on the 

 same subject but in rather a different direction. He has 

 found, namely, that not only shoots, but all plant-organs, 

 whether growing or not, exhibit a daily period of variation in 

 bulk, provided always that the temperature is not too low. 

 The course of the daily period is this, that the organs diminish 

 in bulk (as estimated by their diameters) from early morning 

 until afternoon, when the minimum is reached, and then 

 increase in bulk until towards dawn, when the maximum is 

 attained. 



These variations in bulk depend largely upon the variations 



