The thickness of the zone for the year is rarely equal around the whole circumference of the stem, and this is due to the lesser abundance of leaves on the branches of one side than on the other, or to the prevalence of winds, or some other physical cause, acting in that direction in opposition to the growing process. It should be observed that there is not in timber any appearance of a gradual change from alburnum to perfect wood. On the contrary, in all cases the division is most decided; one concentric layer being perfect wood, and the next in succession sap-wood. The age of trees has been inferred, when a section of the whole stem could be examined, by counting the number of rings of wood which have been deposited around the pith. In tropical countries, however, this method cannot be always relied upon. Woods are variable in quality according to the nature of the climate, and of the soil, as also in a considerable degree to the aspect in which they are situated. Trees grown slowly in open, dry, and exposed situations are more fine and close in their annual rings, and more substantial and durable, than those which are grown in close and shady forests, or rapidly reared in moist or sappy places, the latter being soft and broad in their rings, and very subject to decay; and their pith is not always quite in the centre, for the layers are variable also. The waggon maker takes care to combine toughness and durability by selecting his wood from trees of second growth, or from trees of first growth that from infancy have stood alone, or far apart. If the soft wood trees have stood alone, and are very large (as is often the case with some of the pines), and most of the branches are near the top, the wood near the base of the trunk is sometimes found to be _shaky_. This defect is produced by the action of heavy winds on the top of the tree, which wrenches or twists the butt, and thus cleaves apart the fibres of the wood. If the main-top (_couronnement_, of French writers) of a tree dies while the tree is yet standing, it indicates that water has found its way into the trunk, and that the tree is in a state of decay. The fir which grows on very dry marl, forms very narrow yearly rings; if on rich or damp marl, they are wide; and when on wet soil, they are again smaller. The common fir on moor soil, has even smaller yearly rings than if on dry sand or marl. From this it is evident that too wet or too dry a soil is not suitable for this tree. The alder and the willow grow best on wet soil, and thrive but poorly when standing dry.