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 Orchidese 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 oi Dahlia. 
