8o8 MECHANICS OF GROWTH. 



in length has ceased it is principally the stronger imbibition and swelling of the wood 

 that presses the surrounding layers of tissue outwards and promotes their peripheral 

 growth. 



The intensity of the longitudinal and transverse tensions consequently depends mainly 

 on the addition of water to the turgescent pith and the swelling wood ; any decrease 

 of the turgidity of the pith must cause it to contract, and hence the whole shoot to 

 become shorter and flaccid. This is in complete accord with observation, since withered 

 shoots, /'. e. such as have lost water by transpiration, have not only become shorter but 

 also flaccid. Any diminution of the amount of water absorbed by the wood must in the 

 same manner diminish the transverse tension and the diameter of the shoot. A small 

 loss of water in the peripheral tissue when in a state of passive tension does not on 

 the other hand usually cause directly any considerable increase in its tendency to con- 

 tract ; since the increase in its size due to turgidity and imbibition are generally much 

 less considerable than in the pith and wood. 



If now there are circumstances which cause a daily periodic change in the quantity 

 of water contained in the tissues, the result will be also a periodic increase and decrease 

 in the intensity of the longitudinal and transverse tensions. Such a daily periodicity of 

 the tension has been actually discovered by Kraus (/. c. p. 122), who has observed that 

 the longitudinal tension estimated by the difference in length of the pith and the bark, 

 as well as the transverse tension estimated by the contraction of the bark when detached 

 from woody stems, decrease, under the normal conditions of life, from early morning 

 till midday or early in the afternoon, when they reach their minimum, and then again 

 increase, attaining their maximum early the next morning. Millardet determined this 

 periodicity in quite a different way; and since the objects on which he experimented 

 permitted an exact measurement, he detected in addition an increase, usually small, 

 of the tension in the afternoon. Notwithstanding the statements of Kraus — which 

 are partly opposed to this conclusion, but on the whole confirm it — I am inclined to 

 attribute this periodicity chiefly or altogether to the variation in the amount of water 

 contained in the tissues of the plant at different periods of the day. When transpiration 

 is greatly diminished during the night, the quantity of water in the plant must in- 

 crease, and with this the tension; and conversely the increase of transpiration during 

 the early part of the day must diminish the tension. Space does not permit me to 

 give in detail the opposing statements of other observers ; but this will be done in 

 part further on. Here I need only point out that the periodicity, especially of the 

 longitudinal tension, may possibly be also directly dependent on light, independently 

 of the heat which accompanies the light and of the increase of transpiration caused 

 by it (although this cannot be proved by Kraus's experiments, /. c. p. 125). As far as 

 concerns a daily periodicity independent of temperature, light, and the amount of water 

 contained in the tissues, I could only admit it when any other explanation of the 

 phenomena was shown to be impossible. At present this is not the case. From the 

 intimate dependence and correlation of growth and tension, from the fact discovered 

 by me^ that the daily periodicity of growth in length coincides in every particular 

 with the daily periodicity of tension observed by Millardet and Kraus, and that it is 

 caused simply by changes in temperature and light, I consider it very probable that 

 the daily periodicity of tension is also dependent on these agencies. On the one hand 

 they influence growth and through it the tension, while on the other hand they affect 

 the amount of water contained in the tissues by modifying transpiration and its 

 conduction from the roots. Like all other periodic phenomena of vegetable life, 

 that of tension requires a very careful investigation of its external causes before we 

 resort to the last expedient of assuming internal periodic changes, of which no explana- 

 tion can be given in the present state of our knowledge^. 



^ Arbeiten des Bot. Inst, in Wurzburg, 1872, I, Heft 2. p. 168. 



* [A daily periodicity of thickness in the trunks of trees has been detected by Kaiser (Ueb. die 



