164 MECHANICAL SYSTEM 



the extensibility of bast-fibres and the angle of inclination of the rows 

 of micellae of which the walls are composed. In all fibres which are 

 very extensible, the angle between the direction of the oblique micellar 

 rows (fibrillae) and the longitudinal axis of the cell is very large. 



Ambronn's experiments have shown that the absolute strength 

 of collenchyma is little inferior to that of bast ; on an average 

 collenchymatous walls break under a strain of not less than 10-12 kg. 

 per sq. mm. The essential difference between collenchyma and bast 

 consists in the fact that the limit of elasticity is much lower in the case 

 of the former tissue, a load of 1*5-2 kg. per sq. mm. sufficing to produce 

 a permanent elongation. This physical peculiarity of collenchymatous 

 walls is of the utmost importance in connection with the special 

 function of this tissue. It is only by virtue of this property that 

 collenchyma is able to " furnish mechanical support during intercalary 

 growth, without preventing longitudinal extension." 



In order fully to appreciate the excellent quality of the materials 

 used in the construction of the skeletal system of plants, it is necessary 

 to compare their mechanical constants with those of ordinary cellulose 

 walls. According to Schwendener the tensile strength of the mem- 

 branes of thin-walled parenchymatous medullary and cortical cells of 

 various young Dicotyledonous stems amounts on an average to about 

 1 kg. per sq. mm. There is, therefore, no great risk of error in 

 assuming that the breaking strain never exceeds 1 kg. per sq. mm., in 

 the case of fully imbibed cell-walls composed of relatively pure 

 cellulose and not specialised for mechanical purposes. For the walls of 

 stereides, the breaking strain is from ten to twenty-five times as great. 



The statements which have been made so far apply only to 

 mechanical strands that have been freshly removed from the living 

 plant, and that hence contain their normal proportion of water of 

 imbibition. The elasticity and strength of tissues are naturally affected 

 by the removal of water. Schwendener remarked that the absolute 

 strength of bast increases with diminishing water-content, while its 

 elasticity is correspondingly diminished. More recently Weinzierl has 

 come to similar conclusions on the basis of elaborate researches. The 

 point may be illustrated by an extract from one of Weinzierl's tables, 

 referring to Dasylirion longifolium : 



The physiological interest of these alterations depends entirely 

 upon the circumstance that completely dried-up fibrous strands 

 occasionally occur as constituents of bark. 



