SECONDARY THICKENING. NORMAL DICOTFLEDONS. 471 



from the main axis into the lateral shoots; and in fact each of the distinct layers 

 of any transverse portion is continuous with the equivalent and simultaneously 

 formed layer of the successive succeeding portions ; the cambial layer of a shoot 

 formed in one year, or its yearly production of wood being continuous with the like 

 layers in the next year's shoot, &c. 



The longitudinal course of the single elements, especially of those which are 

 elongated, as appearing in the direction of the 'long grain' of the wood and 

 bast, presents a series of remarkable phenomena, which, in a certain though not 

 strict seiise, are independent of those hitherto discussed. In treating of them we 

 will here leave out of consideration the torsion of the entire masses of wood and bast 

 in twining stems, in so far as this is in immediate relation to the torsion of the whole 

 twining part, as the description of the latter phenomenon forms no part of the 

 present work '. 



In stems with a straight and vertical growth, the elements in question usually 

 have their longitudinal axis in an oblique position, deviating from the vertical, and 

 this is the case both in bast and wood, the latter having been the subject of more 

 exact investigations, which are here to be principally regarded ''. 



The deviation from the vertical is usually less conspicuous in the direction of 

 the radial plane ; although it must take place in this direction in the case of the 

 above-mentioned bundles with elements penetrating irregularly between each other, 

 e.g. in the wood of Fraxinus and Cytisus Laburnum. It is clearly seen, even on 

 observation with the naked eye, in the case of the Guaiacum wood, in which the 

 fibres of successive concentric layers have their ends passing between each other ob- 

 liquely in the radial direction (though to a less degree than in the tangential direction). 



In many woods the oblique position in the tangential plane appears more 

 clearly. As a rule its direction is' the same for all the elements of each concentric 

 layer, and on observation of the surface is indicated by an oblique. ' grain ' or striation 

 running round the whole stem. The angle at which the striae cut the vertical varies, 

 partly according to the species, partly in individual cases. According; to Braun it 

 reaches its maximum — as much as 45" — in Punica Granatum; then follow Sorbus, 

 Aucuparia (up to 40°), Syringa vulgaris (up to 30°), ^sculus Hippocastanum (io°- 

 20°); smaller values are more frequent, e.g. usually 4"- 5°, rarely as much as 10° in 

 Pinus sylvestris, 3°-4° in Populus pyramidalis, Betula alba, &c. 'In many cases,' 

 says Braun, ' especially in Pinus, I have convinced myself that specimens with shorter 

 internodes usually show greater degrees of torsion, than those with longer ones.' 

 The inclination is also said to alter with the age of the tree, becoming greater in the 

 later secondary layers in Punica, and smaller in Pinus. 



The direction of the inclination has been found to be invariably the same in the 

 case of many trees ; right-handed (in the sense of Mechanics) in ^sculus Hippo- 

 castanum, left-handed in Populus pyramidalis. Other trees show one direction as 

 the rule, the other as the exception, e. g. Pyrus communis, Carpinus, chiefly right- 



' On this subject reference may be made to H. de Vries, in Arbeiten des Botan. Instituts zu 

 Wiirzbiirg, Heft III, and the earlier literature on the subject which is there cited. 



'' See A. Braun, Ueber den schiefen Verlauf der Holzfaser, Monatsber. d. Berliner Acad. 7 August, 

 1854; Botan. Zeitg. 1869, p. 747; 1870, p. 158. 



