128 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



In the case of the tendrils and hooks of numerous tropical climbers, 

 contact induces an increased secondary growth by cambial activity, and this 

 is, within certain limits, proportional to the weight supported by them l . 



An increased tension causes in many plants a certain acceleration of 

 growth which may amount to as much as 20 per cent, of the original rapidity, 

 and this is followed, when the tension is suddenly applied, by a retardation 

 of in some cases one-fifth the previous rapidity of growth, and lasting 

 from one to two days 2 . A similar retardation is produced by a sudden 

 increase in the osmotic tension of the cell-walls, and probably the dis- 

 turbances produced by a suddenly increased and prolonged strain act in a 

 similar manner. It is, however, doubtful whether the excessive elongation 

 of such plants as Ranunculus fluitans> Glyceria fluitans^ and Potamogeton 

 natans in rapidly flowing water 3 is solely due to the increased tension 

 acting upon them, for the increased flow of water might act in other 

 ways. 



The stimulus of tension acts locally, and hence new growths formed 

 beyond the point of application of the tension are not affected by it. 

 Apparently tensions of internal origin act similarly to artificially produced 

 ones. Newcombe 4 has also observed that when a stem is enclosed in a 

 plaster-cast the cells of the mechanical tissues remain thinner-walled than 

 usual. The same occurs in the wood-elements even when a rapid transpira- 

 tion-current is maintained through them, and hence it follows that other 

 factors besides the flow of water through them are responsible for the 

 thickening of the walls of the tracheae and tracheides. 



When a shoot is strongly bent the cell-walls on the convex side develop 

 thicker walls because of the tension to which they are subjected 5 . Knight 6 

 observed that in a tree fixed so as to swing in one plane, the annual rings were 

 more strongly developed in this plane, possibly because of the alternating tensions 

 on the two sides. Similarly the pull of the aerial parts upon the roots must act 

 as a stimulus strengthening the latter, and in general the mechanical demand 

 largely determines the degree of development of the organs of attachment. Thus 



under natural conditions, and in that of Helleborus viridis a thick layer of such fibres is normally 

 present. Cf. Kiister, Flora, 1900, p. 173. 



1 Ewart, Ann. du Jard. Bot. de Buitenzorg, 1898, T. xv, p. 187 seq. 



3 The transitory retardation was first observed by Baranetzky, Tagl. Periodicitat d. Langen- 

 wachsthums, 1879, p. 20 (Mem. d. 1'Acad. d. St.-Pe"tersbourg, 7 e se"r., T. xxvn), and more closely 

 investigated by Scholz (Cohn's Beitr. z. Biol., 1887, Bd. iv, p. 323), and by Hegler (ibid. 1893, 

 Bd. vi, p. 383). 



3 Kerner, Pflanzenleben, 1891, Bd. n, p. 495; Goebel, Pflanzenbiol. Schilderungen, 1893, II, 

 P- 33 1 ; Jungner, Bibl. bot., 1895, Heft 32, p. ai. 



4 Newcombe, Effect of Mechanical Resistance on the Growth, 1893, p. 40; Annals of Botany, 

 1894, Vol. vin, p. 234. 



5 Pfeffer, Ber. d. Sachs. Ges. d. Wiss., 1891, p. 642 ; Elfving, Zur Kenntniss d. Kriimmungs- 

 erscheinungen, 1888. 



6 Knight, Phil. Trans., 1803, II, p. 280; ibid., 1811, p. 217 (observations on roots); Biisgen, 

 Bau u. Leben der Waldbaume, 1897, p. 68. 



