MUTUAL PRESSURES OF CORTEX AND WOOD. 575 



transverse tension between cortex and wood by means of detailed investigations ^ To 

 convince ourselves of its existence, it suffices to mark out a ring of cortex from a stem 

 which is growing rapidly in thickness, by means of two superposed annular cuts, and 

 then to make a longitudinal cut in this ring, and separate the cortex from the 

 wood. On now attempting to replace this ring of cortex in its normal position around 

 the wood, it is found to be too narrow, and gapes ; it would require a very powerful 

 force so to extend it that the gaping margins again touch one another, and thus 

 exactly surround the wood. This, however, signifies simply that the cortex was 

 previously compressed radially, and forcibly distended in the tangential direction by 

 the growing wood. 



Gregor Kraus, moreover, recognised 14-15 years ago' that a daily periodicity 

 occurs both in the longitudinal and tangential tissue-tension, such that a decrease 

 of the tensions is noticeable from early morning to mid-day or afternoon, and 

 an increase thence till early morning again. Millardet found and confirmed this 

 periodicity in an entirely different way, which can only be more exactly shown when 

 we come to the consideration of the sensitive organs. It is scarcely possible to 

 doubt that these periodic alterations of tissue-tension depend on the same causes as 

 the similarly periodic variations of growth, the weeping of the root-stock, the daily 

 movements of leaves, &c. 



AU these considerations hold good strictly of shoot-axes, petioles, and the 

 midribs of leaves only. The radical contrast between root and shoot again comes 

 out here; for in the first place it is important to notice that the conspicuously 

 short growing portion — 3-10 mm. long — of normal terrestrial roots exhibits a 

 phenomenon of tissue-tension which differs at least superficially. If the growing 

 portion of a vigorous root is split by means of a longitudinal section, or by two sections 

 at right angles, in the direction of the axis, the parts do not gape concave outwards, 

 and, at first, no separation at all takes place. If roots so treated, however, are left to 

 go on growing in water, the longitudinal halves become curved convex outwards, 

 that is in exactly the opposite manner to those of shoot-axes and leaf-stalks. 

 The phenomenon is explained simply by the fact that the parenchymatous 

 cortex of the root grows more vigorously in length than the axial strand, 

 and that the latter is still highly extensible within the growing region. The 

 tension between the cortex and the axial strand is therefore so small, that the 

 effect in question only makes its appearance on growth occurring subsequently to 

 the splitting. 



The contrast between roots and shoot-axe's, however, is much more conspicuous 

 in parts of the root which are already fully grown. We are here concerned with the 

 fact already mentioned in the lectures on Organography (Lecture II) that the part of a 

 root-fibre which is no longer growing in length may shorten itself very considerably 

 during a long period, so that seedlings may have their stems eventually drawn 

 down into the soil. Referring to what I said in the lecture quoted, we are here 

 only concerned with the causes of this phenomenon itself: this was investigated by 



' The transverse tension between cortex and wood was -first investigated in detail by Gregot 

 Kraus, Bot. Zeii., 1867, p. 113. 



' Kraus (fc. cii., p. I4i) first described the periodic variation of tissue-tension. 



