Ln perennial plants, the tissues which resist climatal change carry on a kind of low vitality, as seen in the trunks of trees, in this country, in the winter. At more favorable periods, these tissues begin to grow in certain directions; buds, and leaves, and new tissues are formed, and deposited, in various parts of the plant, more espe-dally covering the old, and growth or increase is the consequence. Even in plants not producing leaves, this process goes on, and, year after year, new tissues are added to the old. This is especially evident in sea-weeds, which thus exist through very long periods of time. Thus, Professor Schleiden says, "On the great fucus bank of Corvo and Flores, we might yet find, floating about, plants of Sargauum, which had been cut in strips by the bark of Columbus; and in the northern drift, we might expect to discover Lichens that had been transported with the soil in which they grew, from Scandinavia." Nor is this conjecture at all unreasonable, from what we know of the nature of the process of growth in these plants; but we have no means, in cryptogamic plants, of accurately ascertaining the length of time they have been in growing.

Nor is this possible in endogenous plants, or even in all exogens; but, in the latter, the stem presents, very generally, a scries of zones, and each zone has been found to correspond with one period of vegetation. This period mostly represents a year, hence, by counting the number of zones In the trunk of an exogenous tree, we may form an estimate of the years it has existed. It is in this way that the ages of many very old trees have been arrived at. The following list of old trees has been published by Moquin-Tandon, in his Teratologic Vegetale, and is reproduced in the English translation of Schleiden's Principles of Scientific Botany. There are known,Agaveamerican - Endogen.