PRESSURES AND STRAINS IN WOOD AND CORTEX. 577 



chiefly by two factors : on the one hand by the causes of growth which are given in 

 the chemical and molecular structure of the individual cell itself, while on the other 

 hand this individual formative effort can only make itself effective in proportion as 

 the mechanical obstructions permit it. These obstructions, again, are generally of 

 two kinds ; the individual growing cell is prevented from extending equally on all 

 sides by the pressure of neighbouring cells, or it suffers, since it is closely con- 

 nected with its neighbours, passive strains in the longitudinal and transverse 

 direction. How the relative position of an individual cell or cell-layer will 

 determine the effects produced by pressure and strain, may be in general deter- 

 mined beforehand geometrically, with certain premisses; and if it is at the same 

 time borne in mind how growth in any one direction is usually followed by cell- 

 divisions at right angles to it, a deeper insight is thus obtained into the causes which 

 determine the fact that we find the cells on the transverse section of a wood-body 

 arranged in regular radial and tangential rows, and that the forms of the cells and 

 their groupings in the cortex of a woody stem which is growing in thickness must be 

 situated and layered otherwise than before the beginning of growth in thickness, and 

 so forth. Put shortly, ' the plans of transverse sections of stems and roots in 

 particular may be understood geometrically, so far as their histological construc- 

 tion goes, with the aid of the above principles.' This was first done, as regards 

 the structure of wood, by Naegeli ^ and was then treated generally by Detlefsen ^, 

 in his treatise ' Üder das Dicke7iwachsthiwi cylindrischer Organe! Unfortunately I 

 must here pass over the contents of these two unusually instructive treatises, since they 

 could not be presented without digressions, and especially without geometrical figures 

 and mathematical formulae which might not be welcome to the reader of this book. 



Several other facts however may be without difficulty set forth clearly in words. 

 It is certainly not uninteresting to learn that a matter of structure so conspicuous and 

 well known as the formation of the annual rings in wood depends, in the main at 

 least, on alterations of pressure between wood and cortex. I had expressed this 

 hypothetically in 1868, since it struck me that the cortical crevices of the older stems 

 and branches of trees are deepest during the winter and spring, evidently pointing to an 

 increase of pressure between wood and cortex. In the spring, after these crevices 

 have been formed, the tension must thus be diminished ; and at the same time the 

 amount of water used in the unfolding of the leaves may also cause a slight 

 shrinking of the alburnum. With increasing thickening of the wood, however, during 

 the vegetative period, and with more pronounced drying up of the external layers of 

 cortex, the tension between wood and cortex must increase : the layer of wood 

 formed in the spring is therefore produced under diminished pressure, and consists of 

 cells which are larger in the radial direction, while with increasing cortical pressure 

 the extension of the wood-cells and vessels in the radial direction is interfered with, 



* The first geometrical and mechanical treatment of the arrangement of cells on the transverse 

 section of the wood is that of Naegeli, in his treatise ' Über das Dicken7vachsthiiin Jcs Stengels und 

 Anord7iu>ig der Gefäss-st ränge bei den Sapindaceen,^ München, 1864. 



^ Emil Detlefsen, ' Über Dickcnwachsthum cylindrischer Organe' (1878), in Arb. d. bot. Inst, 

 zu Wzbg., B. II. p. 18. The investigations of Detlefsen referred to further on in the text (' Versuch 

 einer mechanischen Erklärung des excentrischen Dickenivachsthums verholzter Axen und JVtirzeln,'' 

 Wismar, 1881) is also found reprinted in Arb. d. bot. Inst, zu Wzbg., B. II. p. 670. 

 [3] Pp 



