iy- 



MECHANICAL LAWS OF GROWTH. 



Fungi which develope among the vegetable mould of woods, and enclose in the 

 margin of their pileus light loosely lying leaves, pieces of stick, and the like. The 

 small pressure from without clearly prevents in these cases the superficial growth of 

 the walls of the cells with which these bodies are in contact, while the adjoining cells 

 extend laterally and enclose them. 



But the most remarkable illustration of this law is seen in the effect produced by 

 a slight pressure on the growth of tendrils, the longitudinal growth of the cells being 

 thus gready hindered and sometimes even stopped, while the cells of the opposite 

 free side elongate rapidly, as is seen even at the first glance without measurement by 

 making a longitudinal section of a tendril curling round a slender support. In what 

 way the slight pressure which acts in a radial direction, and is generally combined 

 with friction, exerts an influence on the longitudinal growth is however entirely un- 

 known. Very similar phenomena are exhibited by the primary and secondary roots 

 of seedlings (as Zea, Faba, and Pisum). If they are allowed to grow in a damp 



Fig. 449.— Growth of the pollen-tube o^ Campanula rapunciiloides : Kp the pollen-grain; 

 ps the pollen-tube closely applied to the stigmatic hair nh. 



locality, and the growing parts are made to press on one side some solid body as a 

 pin or another root, the root bends like a tendril round the body with which it is in 

 contact, this side growing more slowly than the opposite one. It is evidently in 

 consequence of a similar influence of pressure on growth that the aerial roots of 

 Aroideae and Orchideae become closely attached to solid bodies, following exactly 

 their inequalities. But even unicellular tubes, such as the hyphae of Fungi and 

 pollen-tubes (Fig. 449) are induced by contact with a solid body to grow closely 

 applied to it. In this simplest case, where the hydrostatic pressure is uniform 

 over the cell and distends the cell-wall, it does not admit of a doubt that the pressure 

 from without impedes the growth of the cell, independently of turgidity, while the 

 growth proceeds unhindered on the side which is not in contact. 



But the mechanical processes by which pressure on an organ in the radial 

 direction impedes its growth on that side are unknown. The solution of the question 

 must depend in the first place on whether the pressure acts on the cell-wall directly 

 or in some way or other through the protoplasm^ 



^ If the relation between protoplasm and the growth of the cell-wall were better known, stress 

 might be laid on the fact that even a very slight pressure on the cell-wall disturbs the movement of 

 the protoplasm, and may even cause it to become detached from the cell-wall (see Hofmeister, 

 Lehre von der Pflanzenzellc, p. 51). 



