42 EXOGENOUS STEM. WOOD. 



lar, no change in colour takes place; this is also the case in the Chest- 

 nut and Bombax. The relative proportion of alburnum and duramen 

 differs 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, 2f- 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 Lombardy Poplar, in seven 

 years; Robinia, Oak, Scotch Fir, Weymouth Pine, Silver Fir, were 

 decayed to the depth of half an inch in seven years; while Larch, 

 common Juniper, Virginian Juniper, and Arbor Vitas, were uninjured 

 at the end of that time. 



79. 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 cases 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, 99 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 interrup- 

 tion 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 hi 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 dia- 

 meter of the tree. When by the action of severe frost, and 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 injury may be ascertained by counting the number of layers which 

 intervene between the imperfectly formed circle and the bark. In 

 1800, a Juniper was cut down in the forest of Fountainbleau, exhibiting 

 near its centre a layer which had been affected by frost, and which was 

 covered by ninety-one woody layers, showing that this had taken place 

 in the winter of 1709. Inscriptions made in the wood become 

 covered, and may be detected hi after years when a tree is cut down; 

 so also wires or nails driven into the wood. As the same develop- 

 ment of woody layers takes place in the branches as hi the stem 

 of an Exogenous tree, the time when a branch was first given off 

 may be computed by counting the circles on the stem and branch 

 respectively. If there are fifty circles for instance in the trunk, thirty 



