124 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



PART VIII 



THE INFLUENCE OF MECHANICAL AGENCIES ON GROWTH 

 SECTION 35. Mechanical Actions. 



Apart from the fact that a sufficient mechanical resistance renders 

 growth impossible, various stimulatory influences are exercised by tension, 

 pressure, and mechanical vibrations or disturbances. Growing plants are 

 able to overcome considerable external resistances, as when a root or shoot 

 pushes aside large stones, or bursts open masonry into which it has 

 penetrated by some crevice. Under such circumstances, the tension of 

 the cell-walls is gradually replaced by the external pressure, against which 

 the whole of the internal osmotic pressure finally acts when the growing 

 tissue has filled the crevice. Even when external growth has ceased the 

 thickening of the cell-walls may still further increase the pressure which 

 the growing tissue is capable of exerting 1 . 



The observed pressures are, however, not greater than could be 

 produced by the osmotic pressure alone, which usually corresponds to that 

 of from 1-5 to 4 per cent, solutions of potassium nitrate, that is from 4-3 to 

 15 atmospheres, or from 4-5 to 15-5 kilogrammes per sq. cm. In Spirogyra, 

 Chara, and in the root of Zea Mais, the osmotic pressure does not alter 

 when growth is mechanically restrained, whereas in the root of Faba vulgaris 

 it may increase by about one-third its previous value. Even if it only 

 rose in this way to six atmospheres, a 10 cm. thick and 100 cm. long piece of 

 a cylindrical stem or root could support a total external pressure of 6,000 

 kilogrammes. Moreover, when a root flattens itself to the shape of a crevice,as 

 its surface increases or it develops lateral roots, so also does the total pressure 

 it is capable of exerting increase. In small objects the total pressure is 

 not very great, but nevertheless the radicle of Faba can exert a longitudinal 

 pressure of 300 grammes, which is sufficient, when lateral curvature is pre- 

 vented, to enable the apex to bore into a potato-tuber and grow through it 2 . 



The full pressure against a fixed resistance develops with varying 

 rapidity, and usually slower and slower towards the end. The maximum 

 is nearly attained after from two to four days in the case of rapidly 

 growing organs, such as the roots of Faba, but the fact that the investing 

 plaster-cast may only be ruptured after two to three weeks shows that 

 the pressure may undergo a further slow increase during this time. The 

 length of this period is due to the fact that the surface-growth of the cell 



1 Pfeffer, Druck- u. Arbeitsleistungen durch wachsende Pflanzen, 1893. 

 8 Id., 1. c., p. 362 ; Peirce, Bot. Ztg., 1894, p. 169. 



