INTRODUCTION 



the annual rings of tropical trees are difficult to deal with ; they 

 are often entirely absent and no rhythm of structure indicates 

 the periods of growth, and in several instances, to be mentioned 

 later, the pores and other elements steadily increase in size, 

 indefinitely, from the pith outwards to the bark. 



The Heart-wood and Sap-wood so familiar in the Oak, are by 

 no means universal. Familiar examples of trees without 

 heart-wood are the Beech, Sycamore and Birch. American 

 birch, by the way, has both sap-wood and heart-wood. Some- 

 times the two kinds of wood are distinct from each other, as in 

 the Ebony, where the sap is white and the heart black, while in 

 others the former seems to acquire the characteristic colour 

 gradually, and melts insensibly into the darker heart-wood. 



The pith frequently exercises some influence upon the contour 

 of the rings. The common Oaks (Fig. 51) have a five-lobed pith, 

 and the young layers of wood follow it closely, and it is not till 

 the tree has arrived at a considerable age that the influence of 

 the pith is lost. The Hornbeam (Fig. 122) retains it for an even 

 longer period, and instead of the rings becoming gradually more 

 regular the lobed form seems to become more pronounced so 

 that the bark, though externally smooth and cylindrical, is inter- 

 nally thickened where the hollows of the waves occur, and is very 

 thin above their crests. 



The pith of all true Boxwoods is lozenge-shaped in section, or like 

 a narrow bar with a circular swelling in the middle. (Fig. 178.) The 

 West Indian Boxwood, on the contrary, has a round pith that is 

 nearly always cracked, so that this feature is often sufficient to 

 distinguish the true from the false Boxwood. All species of 

 Walnut have a most remarkable pith consisting of a series of 

 diaphragms dividing the tubular cavity into a number of small 

 chambers, as may easily be seen by cutting a twig from a 

 Walnut tree. Workmen are familiar with this rather unwelcome 

 peculiarity, as it forms a serious defect in the centre plank of 

 every log. Although the original pith of the tree-trunk may not 

 often be accessible, yet small knots here and there exhibit this 

 feature. 



The bark has always been considered a most useful aid in 

 identifying woods, and in some cases is the only decisive detail, 

 as in the two species of Eucalyptus, the Morrell and the Salmon 

 Gums of Western Australia. The former has a roughish fissured 

 bark, while the latter has a bark without a fissure or wrinkle, 

 and a cuticle or outside skin with a peculiar greasy lustre, that 

 suggests " at long range " a salmon's scaly skin. Apart from 

 this feature a microscope fails to find a single essential point 

 of difference. 



The terms used to describe the bark, such as corky, wrinkled, 



xvii b 



