64 THE TISSUE-STRAINS 



N. J. C. Miiller even found that a pressure of 13^ atmospheres (14 kgs. per sq. cm.) 

 was required to prevent the pith of Helianthus from expanding 1 . Pronounced 

 strains also exist in motile pulvini 2 , and in the nodes of grasses 5 . 



These high values are hardly surprising, since growing organs may exert 

 equally great pressure upon external objects. Within the plant the stretched 

 tissues correspond to the external resistance, and the whole or a part of the 

 osmotic pressure of the compressed tissues may be brought to bear upon them 4 . 

 The whole of this pressure comes into play in the parenchyma of grass nodes 5 , 

 and in the pith of Helianthus*, whereas in other cases the osmotic tension of 

 the walls of the compressed tissue is only partly neutralized by the counter- 

 action of the passively stretched tissues upon them 7 . 



The History of our knowledge of the strains in tissues is closely connected with 

 the study of certain movements. Thus Duhamel 8 and also Lindsay 9 recognized 

 their importance for particular curvatures, which latter may be very rapid when 

 pre-existent stresses are suddenly brought into play. Dutrochet 10 distinguished 

 between the stresses due to turgor and those due to the antagonism of the tissues, 

 and also recognized that movements and changes in the strains might be produced 

 by alterations of turgor and of the percentage of imbibed water in the cell-walls, 

 as well as by growth. Hofmeister 11 attempted to refer the tissue-strains almost 

 solely to the condition of imbibition of the cell-walls, and undervalued or 

 neglected the importance of turgor. On the other hand, we owe to Hofmeister 

 some valuable conclusions as to the appearance of stresses during the development 

 of growing organs, and as to the distribution of the resulting strains in tissues as 

 well as in cell-walls. Our knowledge was further broadened by Sachs 12 , and more 

 especially by G. Kraus ls , while Nageli and Schwendener have given us a clear view 

 of the general physical principles involved in the correct interpretation of the 

 tissue-strains u . 



strips, and also incorrectly concluded that the Change of dimension on isolation was directly 

 proportional to the previous tension. [The stress and resultant strain are directly proportional only 

 in the case of similar homogeneous materials.] 



I N. J. C. Miiller, Bot. Unters., 1872, Bd. I, p. 53. 



3 Pfeffer, Die periodischen Bewegungen d. Blattorgane, 1875, pp. 105, in. 

 s Id., Dntck- u. Arbeitsleistungen, 1893, p. 401. 

 * Id., I.e., pp. 380, 400, 426. 



5 Id., 1. c., p. 400. 



6 Kolkvvitz, Fiinfstiick's Beitrage z. wiss. Bot., 1897, Bd. I, p. 246. 



7 Pfeffer, I.e., pp. 380, 426; Schwendener and Krabbe, Jahrb. f. wiss. Bot., 1893, Bd. XXV, 

 p. 327 ; Kolkwitz, I.e. 



8 Duhamel, De 1'exploitation des bois, 1764, T. II, p. 479. 



9 Lindsay, quoted in Pfeffer's Physiol. Unters., 1873, p. 3. 



10 Dutrochet, Me"moires, Bruxelles, 1837, PP- 22 5-235- Many of these conclusions were published 

 in detail from 1824 onwards. Johnson (Ann. d. sci. nat., 1835, 2 e se"r., T. IV, p. 321) observed 

 many instances of tissue-strains, but did not interpret them correctly. 



II Hofmeister, Jahrb. f. wiss. Bot., 1859, Bd. 11, p. 237, and 1863, Bd. Ill, p. Si ; Flora, 1862, 

 p. 497 ; Pflanzenzelle, 1867, p. 267 seq. 



12 Sachs, Experimeutalphysiol., 1865, p. 465. 



13 Kraus, Bot. Ztg., 1867, p. 105, and 1871, p. 67; Jahrb. f. wiss. Bet., 1869-70, Bd. VII, 

 p. 209. 



" Nageli and Schwendener, Mikroskop, 1867, i. Aufl., p. 402. 



