EXOGENOUS STEM — ^WOOD. 55 



old trees, there is a marked division between the central Heart-wood 

 or Duramen (durus, hard), and the external Sap-wood or Albvjmvm 

 (albus, white) : the former being hard and dense, and often 

 coloured, with its tubes dry and thickened; while the latter is 

 less dense, is of a pale colour, and has its tubes permeable by fluids. 

 The difference of colour between these two kinds of woods is often 

 very visible. In the Ebony tree, the duramen or perfect-wood is black, 

 and is the part used for furniture, while the alburnum is pale ; in the 

 Beech, the heart-wood is light-brown ; in the Oak, deep-brown ; in 

 Judas tree, yellow ; in Guaiacum, greenish. The alteration in colour 

 is frequent in tropical trees. In those of temperate climates, called 

 white-wood, as the Willow and Poplar, no change in colour takes place ; 

 this is also the case in the Chestnut' and Bombax. The relative pro- 

 portion of alburnum and duramen varies in different trees. Duhamel 

 says that in the oak, six inches in diameter, the alburnum and duramen 

 are of equal extent ; in a trunk one foot in diameter they are as two to 

 seven; in a trunk two feet in diameter, as one to nine. The heart-wood 

 is more useful than the sap-wood, and less liable to decay. The wood' of 

 different trees varies much in its durability. Pieces of wood 2|- inches 

 square, were buried to the depth of one inch in the ground, and decayed 

 in the following order : — Lime, American Birch, Alder, and Aspen, in 

 three years ; Willow, Horse-chestnut, and Plane, in four years ; Maple, 

 Red Beech, and Birch, in five years ; Elm, Ash, Hornbeam, and Lom- 

 bardy Poplar, in seven years ; Robinia, Oak, Scotch Eir, Weymouth 

 Pine, Silver Fir, were decayed to the depth of half an inch in seven 

 years ; while Larch, common Juniper, Virginian Juniper, and Arbor 

 Vitse, were uninjured at the end of that time. 



From the mode in which the woody layers are formed, it is 

 obvious that each vascular^ zone is moulded upon that which precedes 

 it ; and as, in ordinary 6ases, each woody circle is completed in the 

 course of one year, it follows, that, by counting the concentric circles, 

 the age of a tree may be ascertained. Thus fig. 117 represents an oak 

 eight years old, having eight woody layers, b. This computation can 

 only be made in trees having marked separations between the circles. 

 There are, however, many sources of fallacy. In some instances, by 

 interruption to growth, several circles may be formed in one year, and 

 thus lead to an erroneous estimate. Care must be taken to have a 

 complete section from the bark to the pith, for the circles sometimes 

 vary in diameter at different parts of their course, and a great error 

 might occur from taking only a few rings or circles, and then estimating 

 for the whole diameter of the tree. When by the action of severe 

 frost, or other causes, injury has been done to the tender cells from 

 which the young wood is developed, while, at the same time, the tree 

 continues to live, so as to form perfect woody layers in subsequent 

 years, the date of the ipjury may be ascertained by counting the 



