580 LECTURE XXXIII. 



angles to this. The same is of course true also of the diminution of the cortical 

 tension, and this explains the extension of the thickening buttresses which appear 

 at the places of origin of lateral organs. 



' From the changes in dimensions of the cortex during growth in thickness we 

 get the following results : — 



'(i) The tension of a convex piece of cortex is increased by the growth 

 in thickness, that of a concave piece of cortex, on the contrary, is diminished 

 by it. 



' (2) The alteration of cortex-tension produced by the growth in thickness 

 is ccEteris paribus the more considerable the more strongly the cortex is curved. 



' For a convex surface of cortex must become enlarged during growth in thick- 

 ness, a concave one, on the contrary, must diminish, and the changes in size of 

 the surface produced by equal growth are the more considerable the more con- 

 siderable the curvature of the surface is.' 



Further, the changes, hitherto unexplained, in the direction of the branches on 

 a tree-trunk are referred by Detlefsen to this principle ; here, however, it is the 

 weighing down of the branch which depresses it more and more as it increases in 

 length from year to year. ' Since the weight of the branch forces it downwards, 

 it becomes, like any bent rod, stretched on its upper side but compressed below ; 

 the cortical tension of the upper side is thus (and this most evidently at the base 

 of the branch) increased, that of the lower side diminished. The inequality of 

 cortical tension at the insertion of the branch, produced by the weighing down, is 

 often much more considerable than that due to the form of the cortical surface at 

 this place. In the first case the buttresses which run down from the places of origin 

 of the branches spring from their under side, whereas otherwise they run down from 

 above over the two sides of the place of insertion.' For the proper appreciation of 

 these considerations, what has already been said above must of course be borne 

 in mind, namely, that every longitudinal tissue-tension must necessarily induce 

 transverse tensions also, and conversely, every transverse tension must call forth 

 longitudinal tensions. 



Straight branches directed nearly horizontally always show, according to 

 Dedefsen, a promotion of the growth in thickness of their under side, and this 

 is in fact usually most evident at the base of the branch, and gradually diminishes 

 thence towards the apex — branches or stems which stand nearly upright and have 

 their leaves on one side, and which are situated at the margin of a wood, or which 

 by prevalent winds in a certain direction have had the development of their crowns 

 prevented on one side, always show a predominant growth in thickness on the side 

 where most leaves exist. It is often shown externally by the transverse folds of the 

 cortex that it is compressed on the side towards which the unilateral weight of the 

 crown presses down the stem, while the increased tension on the convex side of 

 such objects is betrayed by the smooth surface of the cortex. * Here,' says Detlefsen, 

 * where the inequality of the cortical tension is obvious, the usual explanation, that 

 the excentric growth in thickness is a result of one-sided nutrition, is quite untenable.' 

 ' If the foliage is abundant,' he continues, ' a marked alteration of the form 

 of the bent branch is produced by it. Thus the older branches of orchard trees are 

 seen to be bent by the weight of the leaves and fruit, and to sink more and more the 



