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 diff"erent 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 diff'erent 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 hght, 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. I.e. 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 aflfect 
the amount of water contained in the tissues by modifying transpiration and its 
conduction from the roots. Like all other periodic phenomena of vegetable fife, 
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 Würzburg, 1872, I, Heft 2. p. 168. 
^ [A daily periodicity of thickness in the trunks of trees has been detected by Kaiser (Ueb. die 
