IRRITABILITY. 407 



in the total quantity of the water which the organ contains, 

 and this again depends upon the relative gain of water by 

 absorption and loss by transpiration, assuming that the organ 

 forms part of a plant growing under normal conditions. As 

 regards absorption, we learned in a previous lecture (p. 95) 

 that the root-pressure exhibits a daily periodicity such that 

 the maximum is generally attained in the afternoon, and the 

 minimum about twelve hours later. On comparing the daily 

 period of root-pressure with that of variation in bulk, we find 

 that the two in no wise correspond ; the maximum of the 

 former falls at about the time of the minimum of the latter. 

 As regards transpiration, we have learned in a previous lecture 

 (p. 109) that this process is carried on more actively in light 

 than in darkness, and that a relatively high temperature is 

 favourable to it : it seems therefore possible that the diminu- 

 tion in bulk during the day may be due to the loss of water by 

 transpiration. Kraus has found, as a matter of fact, that the 

 removal of the leaves in the day-time arrests the normal 

 diminution in bulk of the stem. The daily variation in the total 

 quantity is due, then, to the excess of the loss over the supply 

 by day, and the excess of the supply over the loss by night. 



Inasmuch as these variations in bulk are thus dependent 

 upon variations in the quantity of water which the organs 

 contain, and in view of the general similarity between the 

 course of the daily period of variation in bulk and that of the 

 daily period of the tensions, it would seem natural to regard 

 the one as the cause of the other. The change in the diameter 

 of a branch, for instance, is due, according to Kraus, either 

 chiefly to the swelling by imbibition of the walls of the wood- 

 cells, or chiefly to the expansion of the parenchymatous 

 tissues. In whichever of these ways the increase in bulk takes 

 place, it is clear that it must increase the tissue-tensions in 

 the organ ; similarly a decrease in bulk involves a diminution 

 of the tissue-tensions. 



But these variations in bulk are not wholly due to 

 variations in the total quantity of water which the organ 

 contains, but they depend also upon the distribution of the 

 water. Perhaps the most interesting of Kraus' observations 



