8l2 
MECHANICS OF GROWTH. 
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^. 
But in contrast to the phenomena which have now been described, external 
pressure also sometimes causes growth at places where otherwise there would be 
none. Thus Pfeffer has shown ^ that certain hyaline superficial cells on both of 
the flat sides of the gemm^ of Marchaniia possess the power of growing out into 
tubular root-hairs when they remain in contact for some time with a moist solid 
body ; while contact with water produces no effect of the kind. These cells usually 
develope into root-hairs only when their outer surface is directed downwards, while 
those on the upper side, not being in contact with a solid body, do not grow out. 
This, as we shall see presently, is an effect of gravitation, which is however over- 
come by the action of the slight continuous contact, since this causes the cells on 
the upper side of the gemmae also to grow out into root-hairs. The 'haustoria' 
of Cuscuta and Cassytha and the adhesive discs on the tendrils of the Virginian 
Creeper are only formed, as was shown by v. Mohl, on the continuous contact of 
the surfaces of the tissue with a solid body ; and this has been confirmed by recent 
experiments of Pfeffer's {/. c. p. 96)^ In these cases a growth combined with cell- 
division and differentiation of tissue is caused by contact or slight pressure on a 
part of the organ, and would not take place without this pressure. The haustoria 
and adhesive discs thus formed are altogether indispensable for the life of the 
plant; for Cuscuta is nourished exclusively by the haustoria which penetrate into 
the tissue of the host ; and it is by the formation of adhesive discs on the tendrils 
that the Virginian Creeper is enabled to climb up walls. If the tendrils do not meet 
with any solid body to which they can attach themselves by means of these discs, 
^ 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 Tsee Hofmeister, 
Lehre von der Pflanzenzelle, p. 51). 
Arbeiten des Bot. Inst, in Würzburg, Heft I. p. 22. 
3 [See also Darwin, On the Movements and Habits of Climbing Plants, London 1875, p. 84 
et sey.] 
Fig. 479. — Growth of the polleti-tube of Ccrmpajiicia rapicnculoides: Kp the pollen-grain ; 
ps the pollen-tube closely applied to the stigmatic hair nh. 
