62 



THE TISSUE-STRAINS 



the wood-cylinder causes the bark to be passively stretched. Hence an 

 isolated ring of bark shortens, and no longer meets around the stem when 

 reapplied to it (cf. Fig. 15,^). The radial pressure exerted by the stretched 

 bark upon the cambium and wood-cylinder is inversely proportional to the 

 square of the radius, and directly proportional to the intensity of the 

 tangential tension. The wood-cylinders of trees and shrubs are neither 

 compressed nor elongated perceptibly even by the most pronounced 

 tissue-strains, but the pith and other parenchymatous tissues may change 

 considerably in shape when isolated. 



The longitudinal strains in turgid stems and petioles have been investigated 

 more especially by G. Kraus 1 . The following table gives the values obtained 

 by him in growing adult internodes of Helianthus tuberosus^ the measurements 

 being taken immediately on isolation without immersion in water. The changes 

 of length are given in percentages of the length of each internode. 



Transverse strains. The epidermis is under tension in stems and petioles 

 which undergo no secondary thickening, but a shortening of isolated rings 

 of cortex was observed by Kraus 2 to appear first at the points where the 

 internodes begin to broaden. Later this shortening increases in the stem of 

 Helianthus tuberosus to as much as 3 to 4 per cent., and decreases again in 

 the older basal internodes. The strains do not, however, always follow so 

 regular a course. In the case of various woody plants, Krabbe 3 observed a 

 shortening of the isolated bark of from 1-5 to 4-5 per cent, after secondary 

 growth had commenced. 



Similar longitudinal and transverse strains exist in the internodes of 

 grasses 4 , as well as in the motile pulvini of Phaseolus or Mimosa*. Fig. 16 

 represents a median longitudinal slice from the primary pulvinus of a leaf of 



1 Kraus, Bot. Ztg., 1867, and Table i. Also Sachs, Physiology, 1865, p. 468, and Textbook, 

 p. 769. 



3 Kraus, I.e., pp. 107, 115. Cf. also Hofmeister, Pflanzenzelle, 1867, p. 271. 



8 Krabbe, Sitzungsb. d. Berl. Akad., 1882, p. 1,102 ; Wachsthum des Verdickungsringes, 

 1884, p. 8. 



* Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 400. 



5 Millardet, Nouv. rech. sur la pe"riodicite de la tension, 1869, p. 13; Pfeffer, Physiol. Unters., 

 l8 73> ? 1 8, and Periodische Bewegungen, 1875, p. 3. 



