MOVEMENTS OF GROWTH. 369 



the intact strnctures. The pith is therefore in a state of great 

 positive or active strain (compression), the epidermis in a state 

 of negative or passive strain (tension). 



If the length of the uninjured internode is taken as 100, and 

 the change in length of the epidermis and pith be expressed in 

 percentages, we obtain a value (not, it is true, an absolutely exact 

 one) for the intensity of the tension in the uninjured structure. 

 For example, if the total length of an internode used for experi- 

 ment is 50 mm., the length of the isolated epidermis 49, and 

 that of the isolated pith 54, the tension in the uninjured structure 

 would be expressed by the number 10. As a matter of fact we 

 frequently obtain such numbers in experiments, e.g., with inter- 

 nodes of Sambucus nigra. 



It is instructive to determine the tension in the manner above 

 described in the successive internodes of a shoot. If we always 

 refer the change in length of the isolated strips of epidermis and 

 pith to 100, we obtain comparable numbers, and it is then brought 

 out that the tension in the youngest internodes is small, while it 

 rises to a considerable value in those somewhat older, becoming 

 much less again in the still older internodes. Shoots of Sambucus 

 nigra form particularly good material for such investigations. 



With reference to the causes of the longitudinal tension in the 

 internodes, it is to be observed that its origin must primarily be 

 traced back to the strong turgescence of the cells of the pith. The 

 cells of the pith are able to absorb very large quantities of water. 

 The pith consequently endeavours to extend as much as possible, 

 and strives to stretch the extensible peripheral tissues. These, 

 however, are not only extensible, but at the same time elastic, 

 and strive on their part to compress the pith. The high tur- 

 gescence of the cells of the pith further brings about in them 

 specially vigorous growth, a condition which must still further 

 intensify the tension in the internodes. When the pith, with 

 advancing age of the stem, loses its water, and ceases to grow, 

 the longitudinal tension also disappears. In place of it, however, 

 is exhibited, in connection with the vigorous growth in thickness 

 which now goes on, the transverse tension which we have to 

 discuss in 150. 



That the pith does actually possess the power of taking up 

 without difficulty considerable quantities of water, may easily be 

 demonstrated in lecture. In flower stalks of Taraxacum officinale, 

 microscopic examination of a transverse section teaches that 



P.P. B B 



