ELASTICITY AND COHESION OF TISSUES 57 



elasticity. Similarly, many solids undergo slight progressive elongation 

 when heavily loaded for some time. 



The strength of wooden beams and hempen ropes has frequently been 

 tested for technical purposes, but the first determinations of the strength with 

 regard to the actual sectional area of component cell-walls were made by 

 Schwendener and others. These values are, it is true, mere approximations, 

 but suffice to establish the properties discussed above. As instances of a 

 pronounced power of elastic stretching may be mentioned the spore-membranes 

 of Ascobalus and other Ascomycetes, the medullary hyphae of Usnea barbata 1 i 

 the walls of laticiferous tubes 2 , and the cellulose membranes in the expansive 

 parenchyma of the fruit of Impatiens 3 . Growing cell-walls are readily extensible, 

 but their limit of elasticity is low for suddenly applied strains. The modulus 

 of elasticity, however, almost always increases as the membrane becomes adult, 

 so that a greater strain is required to produce the same extension. 



The elasticity and cohesion of the different layers of the cell-wall may 

 differ according to their percentage of imbibed water, and according to their 

 degree of cuticularization, lignification, or infiltration with waxy or mineral 

 matters. When the power of swelling or optical behaviour differs along particular 

 axes, a special micellar arrangement is presupposed, and hence the elasticity 

 and cohesion may be unequal along two or even three axes. In the cylindrical 

 cells of the staminal filaments of Cynareae, for example, the cell-walls appear 

 to be less extensible tangentially than they are longitudinally. At least they 

 retain the same transverse diameter when subjected to an internal rise of osmotic 

 pressure sufficient to cause an elongation amounting to 20 or 30 per cent, of 

 their length 4 . 



SECTION 17. Elasticity and Cohesion of Tissues. 



The elasticity and cohesion of the tissues is not dependent solely 

 upon the properties of the cell-walls, but is also influenced by the turgor 

 of the living cells, as well as by the manner in which the component cells 

 are arranged and joined together. For example, turgid cells and tissues are 

 comparatively rigid, but become flaccid and droop when turgidity is lessened 

 or removed. An increasing external pressure will force more and more 

 water from the turgid cells, which therefore contract to a smaller volume, 

 while at the same time the concentration of the cell-sap increases, and 

 hence the internal osmotic pressure rises. This takes place on the concave 



1 Haberlandt, 1. c., p. 174. 



2 Schwendener, Sitzungsb. d. Berl. Akad., 1885, p. 326. 



3 Eichholz, Jahrb. f. wiss. Bot., 1886, Bd. xvn, p. 561. Further instances are given by 

 Kiister, Sitzungsb. d. Berl. Akad., 1899, p. 825 (Derbesid) ; Nicotra, Sull' elasticita di tensione, &c., 

 1897-8 (repr. from Rend, dell' Accademia di Acireale, Vol. IX). 



4 Pfefier, Physiol. Unters., 1873, p. no. 



