Page:Popular Science Monthly Volume 67.djvu/479

Rh and four thousand years, what a tale they might unfold. Tradition has it that Napoleon encouraged his soldiers before the battle of the pyramids with the picturesque phrase 'forty centuries look down upon you,' and yet the span of a single sequoia about equals what to the biblical chronologies of Napoleon seemed the limit of time. Many of the still vigorous and growing trees sprouted about the time that Christ was born at Bethlehem in Judea. Most of those still standing had commenced to grow at least before the fall of Rome. We can count the annual layers in the wood of those which have been cut down, and calculate with considerable accuracy their age and the varying rapidity of their growth. For instance, the huge section on exhibition at the American Museum of Natural History shows that the climate of California was very propitious about the time that Charlemagne was crowned by Pope Leo on Christmas clay, A.D. 800, as is evinced by the rapid growth of the tree at that time shown by the comparatively thick layer it added to its girth.

It is not strictly correct to speak of these growth layers as 'annual.' They are primarily the result of the varying rapidity of growth of the cells; thus in trees of temperate climes there is a gradual slowing down of vital activity as the summer advances, followed by a prolonged resting period during the winter, and an accelerated resumption of activity in the spring. These varying functions are recorded in the size and nature of the cells formed. For example, in our oak or chestnut the spring wood consists largely of pitted ducts of large size, which are prominent and in marked contrast with the much smaller celled and more solid additions formed by the slower growth later in the season. In cone-bearing trees like the sequoia the differences are almost entirely of size, the transition being abrupt from the very fine wood-cells formed at the close of the season to the much larger cells of the vigorous vernal growth. In the tropics the varying rapidity of growth is not so marked, although here also there is usually a suspension of vital activity during the hot dry season and a vigorous growth during the humid season. This effectually records the alternation of seasons in the rings of growth. It follows that under certain conditions a tree might add more than one ring in a year, but for our purpose, and generally speaking, it is proper to designate these rings as annual. Year after year the sequoias have been adding layer after layer to their girth in ever widening circles. The thousands of tons of bark shed by each tree during its long career, the tens and hundreds of thousands of tons of sap that have coursed through their venerable trunks, and the innumerable progeny of a single tree in the older, more propitious days—a contemplation of these facts assists us in realizing the true proportions of these forest monarchs. Imagination, however, fails