47a SECONDARY CHANGES. 



handed Salix alba chiefly left-handed. Further, it either remains the same in the 

 successive secondary layers of the same stem, or, in many kinds of trees, as Pines 

 and Firs, the direction changes, becoming reversed after a number of similarly 

 inclined layers. 



Among 167 species of Dicotyledonous woody plants and Coniferse, to whigh 

 Braun's investigations extend, the oblique grain is present in in; in the rest, e. g. 

 Pinus Cembra, Populus monilifera, Ulmus campestris and effusa, Fraxinus excelsior, 

 Clematis Vitalba, it has not been observed. 



The arrangement of the fibres in the Guaiacum wood is different from their uni- 

 form obliquity round the entire stem in the trees hitherto mentioned. Here the grain 

 curves backwards and forwards with short undulations, in each layer of wood, often 

 cutting the vertical at 45°, assuming different directions in successive narrow layers, 

 . not in the broad (annual ?) rings. This arrangement, showing a different direction 

 in every small subdivision of the wood, and the interlacing of the elements, 

 in addition to the radially oblique position and interweaving above mentioned, are 

 the causes of the impossibility of splitting the Guaiacum wood in the radial, and the 

 difficulty of doing so in the tangential direction. 



The facts mentioned, and especially the reversal of direction, in successive layers 

 of wood, are sufficient to show, what all accurate investigation confirms, that the 

 oblique grain is a purely anatomical phenomenon, independent of the external con- 

 formation of the plant. It is also only perceptible externally in the case of injuries 

 which lead to the splitting of the tree in the direction of the grain, such as frost 

 cracks, splitting of the cortex in the direction of the grain of the bast, e. g. in Tilia, 

 Syringa, Juniperus, and Thuja, or in the case of an excessive local swelling of the 

 layers of wood, starting from branches or roots, as in many trees (Punica, Car- 

 pinus, Populus pyramidalis) ; this leads to the formation of ridges, which run round 

 the stem obliquely, in the direction of the grain. 



A plausible anatomical explanation of the oblique position of the elongated 

 elements of the wood is afforded in a general way by their conditions of length. As 

 will be shown below, the elongated elements in a number of woody plants increase 

 successively in length for a series of years. As the total length of any portion of the 

 stem remains unchanged during the secondary growth in thickness, and as, further, 

 no enlargement of individual cells at the cost of others which become obliterated 

 occurs, either in the cambium or its products (with the exception of the relatively 

 inconsiderable phenomena connected vrith the expansion of vessels mentioned 

 above), but on the contrary, all the cells of any layer parallel to the periphery 

 grow, and become larger, or at any rate not smaller, it follows that the progressive 

 increase in length of the elongated elements must result in their position becoming 

 oblique ; in the one case this affects the cambial cells themselves, in the other the 

 fibrous elements in process of differentiation. It may at once be added that in the 

 case of stems with the fibres in a tangentially vertical position, as, for example; 

 Fraxinus, the length of those belonging to successive layers must remain the same, 

 or any difference in length must be equalised by radial obliquity only, a point which 

 has still to be investigated. These considerations render the phenomenon intelligible 

 it its main outlines, but by no means explain all the details. It is open to question 

 whether the above-mentioned differences in- length are sufficient by themselves to 



