Now, if we examine the end of our stick more closely we see a series of rings, clearly marked, circling from the center of the tree and ranging in size from the tiny one which encloses the pith, to the large one which forms the outer surface next to the bark. They are caused by a constant annual deposit and outward growth, by which a layer is added to the outer surface of the sap-wood each season. Hence, by counting these we may determine the age of the tree. Less distinct rings may appear but they will not deceive us as we know that they are caused by a cessation of growth, which may have been caused by drouth. As a general rule these rings are more distinct in trees inhabiting a climate where vegetation is entirely suspended by the cold after each layer is formed. In warmer regions they are not so distinct. This is especially interesting when we study fossils of trees which in many cases show a great difference in climatic conditions in the early ages from those we have at the present time. The layers of bark are much thinner than those of the wood and are not so readily distinguished. They are formed from the interior so that the oldest are on the outside. The older ones fall off, however, so that we cannot trace as many rings in the bark as we can in the wood, although one is formed in each for every season that the tree lives. The roots of the tree spread out underground and are the agents through which the tree derives most of the moisture so necessary to its growth. They absorb moisture only at their extremities and usually spread to just such an extent that the water which falls off the outer branches of a tree during a rain, falls exactly where the tender rootlets can gather it up at once and hurry it back up the trunk of the tree. In ground that is springy, or naturally moist, the roots do not depend so much on the rainfall but reach out after moisture wherever it exists in the soil.