54 THE ELASTICITY AND COHESION OF THE PLANT-BODY 



mechanical rigidity, and aids in preventing kinking, especially if a large 

 number of the bundles cross at the nodes. In general the pith is protected 

 from external pressure by the vascular cylinder which encloses it, just as 

 the cortex is frequently shielded from pressure by a hypodermal layer of 

 sclerenchyma. In addition, the phloem is often covered by an outer layer 

 of pericyclic sclerenchyma, while in the lime the masses of hard bast- 

 fibres form a kind of skeletal framework in whose meshes the soft phloem 

 tissue is sheltered. 



For further details reference may be made to Haberlandt's comprehensive 

 summary, and to the original paper by Schwendener *. Branches and petioles are 

 usually stronger towards their bases, where the greatest leverage is exerted, and 

 this increase in the case of the haulms of grasses and sedges may very closely 

 approximate to the required distribution of strength. In many cases, as in the 

 leaves of Phormium tenax, longitudinal foldings or other changes of shape aid 

 in conferring the required rigidity. Stahl and also Wiesner 2 have found that 

 certain peculiarities of structure are for the purpose of avoiding the injurious 

 effect of storms of wind, rain, or hail. 



Etiological explanations do not, however, afford any insight into the 

 physiological processes involved. The latter may be influenced by the external 

 conditions, and these within certain limits also affect the rigidity of the plant. 

 Thus the cell-walls are thinner and weaker in etiolated plants than in normal 

 ones, and the strength of the plant and of special organs is partly dependent 

 upon the demands made upon them. This power of response is very marked 

 in the petioles of Helleborus m'ger, which can normally bear a load of 400 grammes, 

 but may support a weight of 3,500 grammes if the load is very gradually increased. 

 This is owing to the fact that certain thin-walled cells in the leaf-stalk develop 

 into a pronounced ring of sclerenchyma 3 . 



SECTION 16. Elasticity and Cohesion of the Cell-wall. 



The great differences in the rigidity and elasticity of different cell- 

 walls are mainly the result of their specific molecular structure. Thus 

 certain bast-fibres whose walls are composed of cellulose have a tensile 

 strength equal to that of wrought iron (13 kilogrammes per sq. millimetre), 

 or even of steel (25 kgs.) 4 , whereas the elastic limit of the walls of parenchy- 



1 G. Haberlandt, Physiol. Anat., 1896, 2. Aufl., p. 134. Cf. also Detlefsen, Arb. d. Bot. Inst. 

 in Wiirzburg, 1884, Bd. Ill, pp. 144, 408, and the criticism of this work by Zimmermann, Bot. 

 Centralbl., 1884, Bd. XIX, p. 149; Schwendener, Das mechan. Princip im Bau d. Monocotylen, 

 1874. 



a Stahl, Regenfall u. Blattgestalt, 1893, pp. 149, 170; Wiesner, Ann. dn Jard. bot. de 

 Buitenzorg, 1897, T. XIV, p. 283 sq. 



3 For instances of the influence of pressure and strains upon the thickening and strength of the 

 tendrils and hooks of tropical climbers see Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, T. XV, 

 PP- 193-195,213, 217, 235. 



* For details see G. Haberlandt, Physiol. Anat., 1896, 2. Aufl., pp. 143, 121. Cf. also 



