If we begin an examination of the bark of Hevea from the outside of the tree, we find first a brown or grey layer of cork, which is generally thin in young and untapped trees. The function of the cork is purely protective. Secondly, beneath the cork there occurs in healthy trees a thin dark green layer of living cells. Discolouration of this layer may be taken as indicating that the health of the tree requires attention. Thirdly comes a granular yellowish or pinkish layer of tissue, which makes up the greater part of the thickness of the bark. The granules consist of groups of thick-walled stone cells. Owing to the small size of these groups, they offer little obstruction to the passage of a tapping knife, but they quickly turn the edge of a razor, making the preparation of microscopic sections of the bark a matter of some difficulty. The granular layer passes insensibly into a very soft white layer, which immediately adjoins the wood. These four layers make up the bark as popularly understood. If the bark is stripped from the tree, separation takes place at the cambium, which is destroyed in the process of stripping.