34 THE MECHANICS OF GROWTH 



position of the nucleus, as well as the frequent accumulation of the protoplasm 

 at the growing apices of hairs, pollen-tube, and fungal hyphae, are not merely 

 the direct result of the processes involved in growth *. 



The above remarks apply to each individual cell in a tissue, although 

 here the interactions of the cells upon one another must also be considered. 

 Of the purely mechanical actions which act in a self-regulatory manner, 

 it may be mentioned that the increase in size of the more rapidly growing 

 cells and tissues is retarded by their attachment to or enclosure by less 

 rapidly growing ones. As growth continues a portion of the osmotic 

 pressure of the growing cells acts against this resistance, and only the 

 remaining portion is available for maintaining the stretching tension in 

 the growing cell- walls. Hence the retarding tissues become stretched 

 and are under tension, while the actively growing ones are compressed, 

 and are unable to grow when the pressure rises above a certain limit 2 . 

 Otherwise, however, the associated tissues, in the absence of any sliding 

 displacement, grow at equal rates, although the walls of the cells subjected 

 to pressure may be hardly stretched at all, while those under tension 

 may be stretched to a very marked extent. 



Growth may overcome a considerable external resistance, as is shown 

 when a root or stem bursts a plaster cast in which it has been imbedded, 

 or when an actively growing tissue stretches a neighbouring one beyond 

 its limit of elasticity and causes it to rupture. The latter frequently 

 happens during development, and since it is the direct result of growth, 

 it is only produced when the latter is active and hence does not occur 

 in the absence of oxygen. 



Mechanical ruptures occur during the cracking of the bark of trees, and 

 they involve living as well as dead tissues during the outward growth of endo- 

 genous organs. The splitting of the pith in the hollow stems of Umbelliferae 

 and Gramineae is the result of the active tangential growth of the peripheral 

 regions 3 , and the stems remain solid when the enlargement of their diameters 

 is prevented by imbedding in a plaster cast 4 . Frequently also the spiral 

 tracheides of the primary wood are torn asunder during the growth in length of the 



1 Berthold, Protoplasmamechanik, 1886, p. 267 ; Klebs, 1. c., p. 508 ; Reinhardt, Jahrb. f. wiss. 

 Bot., 1892, Bd. xxin, p. 498. Such accumulations appear as secondary phenomena of geotropic 

 and other curvatures, but rapid growth may occur in their absence. Sokolowa (1. c., p. 87) discusses 

 the influence of the direction of protoplasmic streaming. Cf. also Ewart, The Physics and 

 Physiology of Protoplasmic Streaming, Clar. Press, 1903, pp. 34, 55. 



3 Pfeffer, Druck- u. Arbeitsleistungen, 1893, pp. 380, 426. The growing zone shortens when 

 the apex of a root is fixed in a plaster cast. On the resumption of growth when the object is 

 liberated cf. Pfeffer, 1. c., p. 351. 



3 Cf. de Bary, Comp. Anat. (Clar. Press), 1884, pp. 214, 215, 557; Harting, Linnaea, 1847, 

 Bd. xix, p. 553- 



* Newcombe, Annals of Botany, 1894, Vol. VIII, p. 403 ; Bot. Gazette, 1894, Vol. xix, p. 149. 



