As spring succeeds winter the inactivity of the plant-machine is followed by a period of energetic life; opening buds and elongating shoots create a demand for a plentiful supply of ascending sap, and in response to this the tree produces a fresh cylinder of wood composed of relatively wide conducting tubes. After the first rush of life is succeeded by a phase of more uniform and gentler activity, the demand for water becomes less exacting and the wood which is formed during the rest of the growing season consists of narrower water-pipes. A period of rest ensues, until in the following spring new layers of larger tubes are laid down in juxtaposition to the narrower elements of the latest phase of the preceding summer. This alternation of larger and smaller tubes produces the appearance of concentric rings on a cross-section of a tree. It is not the pause in the active life of the plant which is responsible for the effect of rings, but the fact that the wood produced immediately before and immediately after the pause is not structurally identical. In trees grown under the more uniform conditions of certain tropical regions, the annual rings are either feebly developed or absent; for example, in some Indian oaks the wood shows no concentric rings of growth. Stated in general terms, rings of growth in the wood of a tree are the expression of a power possessed by the plant of regulating the structure of its component elements in response to the varying nature of the external stimuli. In certain circumstances, for example after the destruction of the young buds by caterpillars, the tree makes a special effort to repair the loss by producing a new set of shoots. This may be recorded by the occurrence of two concentric rings in one season. An extreme instance of departure from the normal has recently been described(6) in which a tree of _Theobroma cacao_ (the cocoa tree), planted in Ceylon in the summer of 1893 and felled in January 1901, after a life of just over 7 years, was found to have 22 rings in its stem. In this case the tree shed its leaves three times a year, and each break in the uniformity of its vital activities was registered by the apposition of what under ordinary conditions are spoken of as spring and late-summer wood. At Aden trees stated by natives to be very old showed only five or six rings of wood, a fact connected with the almost complete lack of rain and with the uniform conditions of existence.