CARBOHYDRATES, CHITIN AND CUTIN 



289 



which resists any such tendency. The microfibrils of the cellulose 

 frame, which encircle the cell horizontally to obliquely, have con- 

 siderable tensile strength which is comparable to that of bast fibres 

 and is due to primary valency bonds. In the axial direction, however, 

 these fibrils are held together only by interfibrillar substances of much 



-2r- 





fa tit.* itit' 



Pa 



O) 



b) 



Fig. 143. Wall tension in cylindrical cells, a) Anisotropy of the strength F and of the 



wall tension p axially (index a) and tangentially (index t) ; b) derivation of longitudinal 



(Pa) and lateral stress (pj). 1 length, r radius of the cell, d thickness of the cell wall. 



weaker solidity. Consequently, a cylindrical cell of tubular texture has 

 less strength axially than tangentially (Fig. 143^)- It is therefore not 

 difficult to understand that the elastic extension by the turgor occurs 

 preferentially in the axial direction. 



The turgor tension in the cell wall likewise differs according to the 

 direction, and in the same sense as the strength of the wall. As the 

 equation (Castle, 1937b) wall tension p X cross section of wall = 

 turgor pressure T x liquid cross section applies, we have 



p3-(2 7rrd) = T-TiT- 

 pr(2ld) =T-2rl 



for the axial (pj and tangential (p,) wall tension, where d is the wall 

 thickness, r the radius and 1 the length of the cylindrical cells (Fig. 

 143b). The resultant ratio of p^ to p^ is 2:1, i.e., the tangential 

 wall tension is double the axial wall tension. Although the lateral 

 stress in the extending cell is twice the longitudinal stress, it grows 

 in length only. This is possible if the F^: F^ strength ratio is above 2, 

 as there is every reason to think it will be, since primary valence bonds 

 are chiefly responsible for Ft, whereas cohesive forces, which are ten 

 times smaller, determine F^^ (see Tables III, p. 31, and FV, p. 32). 



