MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 8ll 



prevented by the cells which have now been removed. When once projecting 

 beyond the cut, they grow more rapidly than before in a lateral direction in conse- 

 quence of the turgidity, and become divided by transverse and longitudinal walls. 



The further development of such a callus where branches have been cut off 

 leads to the well-known overgrowth on the stumps. In internodes of seedlings of 

 Phaseolus which had accidentally become hollow, I found the medullary cells which 

 surrounded the cavity to have grown into it in the form of spherical or club-shaped 

 papillae; divisions ensued, and nuclei were formed in the cells thus produced. The 

 medullary cells which exhibited this active growth on the free surfaces of their walls 

 would have retained their polyhedral form had the pith remained solid, because 

 every surface of the cell- wall would have been exposed to the pressure of the two 

 adjoining cells ; but in consequence of the formation of the hollow, the pressure was 

 removed on one side, and the turgidity, being no longer neutralised, caused the cell- 

 wall to swell out, and induced in it an active superficial growth ^ These phenomena 

 and others of the same kind show that it is often sufficient merely to remove the 

 pressure to which tissues or individual cells are subject in order to bring about an 

 active growth of the free surfaces of their cell-walls. The first cause at least of 

 the new growth is the distension of the free surfaces of the cell-walls in conse- 

 quence of the turgidity of their cells which was previously neutralised by that of the 

 adjoining cells. But that a very small pressure from without is sufficient to prevent 

 the growth of softer tissues at the points of contact is seen in the case of many large 

 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 greatly 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 

 unknown. Very similar phenomena are exhibited by the primary and secondary 

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

 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 

 Aroidese 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. 479), 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 



Prantl succeeded in artificially inducing similar phenomena in the tubers of Dahlia. 



