INTRODUCTION 



the living condition, the gaps made in the tissue are promptly filled 

 up with a brownish mass of cells almost as soon as they are made. 

 (Figs. 124 and 186.) Whether these cells be Thyloses (see p. xx), 

 or Callus, I am not in a position to decide. Nordlinger holds them 

 to be the former, but it seems difficult to believe that the large 

 spaces sometimes found can be filled up in this way. Why, too, 

 is the colour always changed to brown ? This needs further 

 investigation. The resulting brown flecks are as characteristic 

 of the species as any other feature, because the grub is faithful 

 to its special tree and prefers it to all others, therefore as a series 

 of years can scarcely pass without the presence of the grub, no 

 log of wood of a susceptible species of tree will be found without 

 these traces. A section of English Birch will often display these 

 ' flecks ' in hundreds. A peculiarity of the flecks is their property 

 of giving rise to new rays in abundance, sometimes amounting 

 to a dense brush-like tail (Mountain Ash, Maple). Similarly the 

 " Bird's-eye figure " of Maple arises from the attacks of an insect. 

 In this case the repair of the injury causes a little dimple-like 

 depression in the outside of the wood, which is faithfully overlaid 

 by every subsequent annual ring. The dimpkis therefore a min- 

 iature hollow, containing a number of strata, and if the wood be 

 sawntangentially, the top of the hollow is removed and the edges 

 of the successive strata exposed in concentric circles, resembling 

 the contours on a geological map where a bend in the strata has 

 been denuded. (Fig. 170.) The tails or brushes of rays may 

 be seen in either radial or transverse sections, thus indicating 

 their origin, even if the seat of injury be not evident in the 

 specimen examined. 



Having dealt with the general structure of wood as it may be 

 seen with the naked eye, it is necessary to consider the form in 

 which the elements are arranged. It is not the intention here 

 to go into microscopical details concerning the cells themselves. 

 That portion of the subject has already been dealt with in 

 many excellent text-books, and as the grouping or arrangement 

 of the elements of the wood, rather than the cells, pores, etc., 

 themselves is the material upon which this work is based, it is 

 unnecessary to enter upon details which are so readily accessible 

 elsewhere. Moreover, to study the histology of wood, it is 

 necessary to adopt laborious methods, while here it is desirable 

 to employ simple means, and to do as much work as possible 

 with the naked eye or the pocket lens. The microscope is a 

 valuable and even indispensable accessory, but even here low 

 powers of magnification meet all needs, and more is lost than 

 gained by restricting the field of vision : no lens should be used 

 which will not bring the whole breadth of an annual ring into 

 view at the same time. 



xix 



