EFFECTS OF CORTICAL PRESSURE, ETC. 579 



such a manner that their lower part (three feet) was rendered immovable, while 

 the upper part of the stem with the crown could bend under the pressure of the wind. 

 During the period of vegetation the upper movable part of the stem increased con- 

 siderably in thickness, the lower immovable part much less so ; this is easily 

 explained by the fact that the bending to and fro of the upper part of the stem, 

 under the influence of the wind, causes the cortex to be extended on the convex 

 side each time and so loosened, and the pressure thus diminished. The growth of 

 wood in a young tree which was free to move under the pressure of the wind 

 exclusively north and south, behaved in a corresponding manner : in this north 

 and south direction the growth of wood was stronger, and stood as regards that 

 formed east and west as 13:11. Knight himself, however, gave an incorrect 

 explanation of the phenomena observed by him. 



The promotion of the growth of wood by means of longitudinal incisions in the 

 cortex of the stems of young trees, continued from the crown to the root, has 

 also been employed for a long time practically, and,, as I have convinced myself for 

 some years past, with the best results : the stems grow more rapidly in thickness, the 

 thicker alburnum can convey more water containing nutritive materials from the 

 roots to the crown of the tree, and thus promote assimilation, and this in its turn 

 again promotes the formation of wood in the stem. 



We owe a new contribution to our knowledge in this direction to an 

 investigation by Emil Detlefsen (1881), who first pointed out the fact that in 

 stems and branches with excentric layers (e. g. of the Pine, Maple, Walnut, Vine), 

 and therefore where, as in Fig. 318 above, the annual rings are thicker on the one 

 side than on the other, the formation of cortex is also more energetic on the 

 side of the stronger growth of wood, and the thickness of the cortex more 

 considerable. The necessity of this is at once obvious, since the pressure 

 which the cortex exerts on the wood re-acts also conversely from the wood on 

 the cortex. Detlefsen's remarks on the very commonly occurring causes which 

 must effect a diminution or increase of cortical pressure are particularly valuable 

 however. Wherever a branch arises on a stem, and at the places of origin of the roots, 

 an increased growth in thickness is found. From the places of origin of the thicker 

 branches descend buttresses of the stem, it may be metres long ; and in like manner 

 we find at the lower end of the stem the places of origin of the large laterally 

 spreading roots made recognisable by thick buttresses running upwards. On the 

 branches, strongly thickened places are found always and only running in the direc- 

 tion towards the roots, and on the roots, on the contrary, in the direction towards the 

 stem. Above the places of origin of the branches moreover, and laterally from these, 

 increased growth in thickness occurs, and conversely in the roots. The explanation of 

 this universally observable fact is, according to Detlefsen, a very simple one. 'By 

 means of a branch growing in thickness, and by means of a lateral root behaving 

 in the same way, the cortex of the organ whence they spring is pushed apart and 

 the cortical tension thus diminished, and this naturally induces an increased growth 

 in thickness. The extension of the thickening buttress also is very easy to 

 understand if tlic anatomical constitution of the cortex is borne in mind. It is 

 clear that a pull acting on the cortex must make itself noticeable over much greater 

 stretches in ihe direction of the course of tlie bast-fibres than in the direction at right 



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