MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 729 



from wcody stems, decrease, mider 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 attri- 

 bute 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, i. e. independent 

 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 coincides in every particular with the daily 

 periodicity of tension observed by Millardet and Kraus, and finally from the fact that 

 the periodicity of growth is caused simply by changes in temperature and light, I con- 

 sider 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 explanation can be given in the present state of our knowledge. 



Sect. 16. — Modification of Growth caused by Pressure and Traction. 



Cells or whole masses of tissue may be subjected to pressure and traction in very 

 different ways. On the one hand these forces may result, in a perfectly normal 

 manner, from the tension of the tissues; on the other hand, external and more 

 accidental circumstances may cause single cells or masses of tissue to be com- 

 pressed or stretched in particular places by solid bodies, or tissues to become 

 accidentally freed from the pressure and traction to which they are normally subject. 

 The numerous phenomena which indicate or prove that growth is altered in this 

 way have however at present been exactly investigated from this point of view in 

 only a few cases. The following will therefore only serve to draw attention to a 

 subject in which further discoveries must contribute to the establishment of a 

 mechanical theory of growth. 



I. Every cell-wall is subject to Pressure from within, by which it is distended, 

 so long as the cell is turgid. But since the daily experience of microscopists 

 teaches us that all growing cells are turgid; and that on the other hand no cell 

 which is unable to become turgid in consequence of openings in its cell-walls 

 has any power of growth ; and that moreover withered internodes, leaves, and roots 



Arbeiten des Bot. Inst, in Wiirzbiug 1S72, Heft IT, p. 168. 



