CHAPTER IV 



THE ELASTICITY AND COHESION OF THE PLANT-BODY 



SECTION 15. Mechanical Strength and Rigidity. 



THE plant and all its organs must be adapted to the mechanical 

 demands made upon them. Thus, the stem of an oak must not only be 

 able to firmly support its crown of foliage, but must also be strong enough 

 to withstand the leverage exercised by strong winds. Even young oak 

 steins are, however, not nearly so much bent as are the slender haulms 

 of rye, which at once become erect in calm pauses, even though previously 

 the ears touched the earth. In spite of its great power of bending, the rye 

 haulm possesses considerable rigidity, for if the basal portion is fixed 

 horizontally, the free portion is not only able to bear the weight of the ears, 

 but also to raise them by performing an upward geotropic curvature, 

 although the basal region may then be required to withstand a shearing 

 stress of 5 kilogrammes. A long grass-haulm, which is about 400 times longer 

 than its diameter, is a much more slender structure than a rigid factory 

 chimney. The latter is indeed not nearly so resistant as the trunk of an 

 ordinary tree, for if the crown of foliage from a tree of corresponding size 

 were fixed to a factory chimney, a gentle breeze would cause the latter 

 to snap across. In the crown itself the base of a moderately large branch 

 is frequently subject to a shearing stress of 5,000 kilogrammes *. Less 

 rigidity is required in roots, but they are often subjected to considerable 

 tension when the stem, bent by the wind, tends to drag them out of the 

 ground. Similarly tendrils and the stems of climbing plants have con- 

 siderable tensile strength, but usually a relatively small degree of mechanical 

 rigidity. The maximal tensile strain for a square stem of Uncaria with a side 

 of 5 mm. may be as much as 90 kilogrammes (360 kgs. per sq. cm.), and for the 

 flexible bases of attached tendrils of Stryclmos laurina 2 mm. in diameter 



1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 410; Meischke, Jahrb. f. wiss. Bot, 1899, 

 Bd. xxxni, p. 359. [The following definitions may render the text clearer: the two factors 

 concerned in the elasticity of a body are (i) its compressibility, (2) its rigidity. The compressibility 

 may be defined as the ratio of the cubical compression per unit volume to the pressure producing it, 

 and it is therefore the reciprocal of the volume-elasticity. Rigidity is dependent upon the power 

 of resisting external forces (shearing stresses) which produce change of shape without change of 

 volume. A tensile stress upon a root may not only alter its shape if it elongates, but also its volume. 

 The power of resisting tensile stress is therefore dependent upon the volume-elasticity as well as 

 upon the rigidity, both of which again depend upon the degree and mode of cohesion between the 

 non-homogeneous materials of which the root is composed.] 



