“The stalk is the common support of the plant’s various parts. It is called annual or herbaceous when it lives only one year, as in the potato, spinach, parsley, and all forms of vegetation that from their soft structure belong to the class of herbs. Ligneous is the name given to the stalk when, designed to live for a greater or less number of years, it is made of strong woody fibers, such as we find in the trunks of trees. “Let us make a clean cut through any tree-trunk, that of an oak for example. We shall find it divided into three parts: in the center the pith or marrow, very slightly developed; around the marrow the wood proper; and, finally, on the outside, the bark. A closer examination shows that the wood is formed of concentric layers which are indicated in the cross-section by a series of circles having the marrow for a common center. These layers are called ligneous zones or, since one is formed every year, annual layers. During the summer there is a downward flow, throughout the tree, of a peculiar liquid, the descending sap, which constitutes the fluid nourishment of the tree. This liquid runs between the wood and the bark and becomes, little by little in its course, on one side a layer of wood which attaches itself to the outer surface of the preceding year’s layer, and on the other side a thin sheet of bark which is added to the inner surface of the bark already formed. “Thus each year both bark and wood form a new layer; but this added layer is applied in opposite ways in the two instances,—outside on the wood, inside on the bark. The wood thus encircled from year to year by new layers increases in age toward the center and becomes younger and younger toward the circumference, whereas the bark, lined every year with a fresh sheet, shows its youth on the inside and its age on the outside. The first buries inside the trunk its decrepit and dead layers; the second thrusts its old layers outside, where they crack and fall off in large scales. This aging process is simultaneous on the outside and in the center of the tree-trunk; but between the wood and the bark life is always at work, creating fresh accretions. “Here are some experimental proofs of this annual formation of a ligneous layer. A strip of bark is removed from the trunk of a tree, and on the wood thus laid bare is fastened a thin sheet of metal. The bark is then replaced and bound with ligatures so that the wound may heal. We will suppose ten years have passed. The bark is raised again at the same place. The metal sheet is no longer visible; to find it you must bore deep into the wood. Now, if you count the ligneous layers removed before reaching the metal sheet, you will find precisely ten, just the number of years that have passed.