SECONDARy THICKENING. NORMAL DICOTYLEDONS. 507 



fibres always increase in length. The cells of the bundle-parenchyma show no 

 change. The members of vessels and the tracheides behave differently according to 

 the kind of wood. 



By way of example we may reproduce Sanio's statements regarding the mean 

 length of the elements in a stem of Quercus pedunculata 130 years old, with very 

 narrow annual rings. The dimensions are expressed in millimetres. 



An increase in the width, and in the thickness of the walls of the elements, 

 accompanying the increase of the latter in length, is likewise evident in many 

 Dicotyledonous woods, especially in the case of the large vessels of the spring-wood. 

 Quercus pedunculata is a typical example of this. Sanio found the mean radial 

 diameter of these vessels in the third annual ring =o-o8™™; its definitive dimension, 

 which is not attained before the sixth, year, amounts to 0-31 — o-ss""™. As regards 

 the change in the definitive constant length of the elements at different levels 

 of the stem, only one investigation, on a single Birch-stem, exists, which may be 

 referred to in Sanio's treatise. 



The increase of the ligneous elements in length and width is, in some of the 

 cases observed, the immediate consequence of a corresponding successive enlarge- 

 ment of the cambial cells ; while in others this is not so. The former is the case 

 in the Coniferae, where both the successive enlargement of the cambial cells, and the 

 relatively trifling increase of size, which the ligneous elements undergo after their 

 origination, are distinct. In the Pine, for example, the tangential diameter of the 

 winter cambial cells is more than twice as great (o'02 6™«») in a stem a hundred years 

 old as in a shoot one year old (0.012™'"); in a one-year-old apical shoot of the 

 same species, on the other hand, the length of the cambial cells is 0-87™"!, that 

 of the tracheides in the autumn-wood 1-05™"'. The other category includes, 

 for example, Rhamnus Frangula, Cytisus Laburnum, Caragana arborescens, and 

 probably most Leguminosse. As far as the investigations extend the cambial cells 

 here undergo little or no increase of size, as their distance from the pith becomes 

 greater. The increase in size is thus due to the ligneous elements themselves, 

 which have already been developed from the cambium; the woody fibres, which 

 are especially affected by it, become in Cytisus Laburnum, for example, six times 

 as long as the cells of the cambium. 



Sect. 154. In many woody plants there further arise those differences of physical 

 properties between successive zones of growth or annual rings, on which is founded 

 the technical distinction between sap-wood (Splint, Alburnum, Aubier) and duramen- 

 ox ripe-wood (Kern, Herz, bois parfait). The sap-wood is wood which has been 

 completely developed from the cambium, and which possesses the anatomical 

 characteristics above described, and the physiological properties which depend on 

 them. It has a light whitish or yellowish wood-colour. In many trees, as for 

 example Acer pseudoplatanus, platanoides, and Buxus, the alburnum condition 



