8i2 



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 gemmae of Marchantia 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 eff'ect 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' 



Fig. 479.— Growth of the polleti-tube of Ca7npanula rapunculoides : Kp the pollen-grain ; 

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



of Cuscuia 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 Pfeff'er'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 Cuscuia 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 fsee Hofmeister, 

 Lehre von der Pflanzenzelle, p. 51). 



"^ Arbeiten des Bot. Inst, in Wiirzburg, Heft I. p. 22. 



3 [See also Darwin, On the Movements and Habits of Climbing Plants, London 1875, p. 84 

 et scy.] 



