MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION, 733 



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 eifect 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 ex- 

 periments of Pfefifer's {I.e. 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. These 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, 

 they dry up and fall off, while those which have formed discs increase in thickness 

 and become woody. 



The injurious effect on growth of an external pressure on the cells is very evident 

 in the formation of the annual rings in wood. In the earlier editions of this work 

 I called attention to the fact that the larger radial diameter of the wood-cells in the 

 portion of the rings formed in the spring, and their smaller radial diameter in the por- 

 tion formed in the autumn, may possibly depend on a difference in the pressure from 

 the surrounding bark to which the cambium and the wood are subject, this pressure 

 being less, as we have showm, in the spring, and constantly increasing during the sum- 

 mer. This hypothesis has been fully confirmed by H. de Vries's recent investigations^. 

 In branches two or three years old he increased the pressure of the bark in the spring 

 by firmly winding strings round them at particular places. ' The experiment showed in 

 all cases, firstly, that the absolute thickness of the annual ring was less beneath the liga- 

 ture than the mean thickness of the same annual ring at some distance above or below 

 that spot. In several instances the difference was so considerable that the spot where 

 the experiment was made appeared of considerably less diameter even to the naked eye, 

 and this effect was increased by the formation of cushions of wood immediately above 

 and below the ligature. Secondly, the absolute thickness of the ' autumnal layer ' of 

 wood (up to the middle of August, when the increase in diameter of the tree on which 

 the observations were made ceased), was always greater, and generally considerably so, 

 than the normal thickness at the spot where the experiment was made. In the trees 



* Aibeiten des Bot. Inst, in Wiirzburg, Heft I, p, 22. 



^ [See also Darwin, On the Movements and Habits of Climhing Plants, London 1865, p 84 

 et seq. — Ed.] 



' H. de Vries, Flora 1872, No. 16. 



