12 THE LIFE HISTORY OF A TREE 



into wood is greater than that outwards which turn into 

 bark, and thus the layer of wood formed is much thicker than 

 the layer of bark. In this way each year a new layer of wood 

 is added to the outside of the older wood, and a new layer of 

 bark to the inner side of the old bark. The result of this is 

 that the outer bark gets stretched until it can stretch no more, 

 and then it cracks or peels. Before this stage is reached, 

 however, another layer of cells in the bark is formed, called 

 cork cambi^^'^n, and this continuously forms cork on the outer 

 side. The whole tree is thus encased in cork, which prevents 

 water getting into the tree when the outer bark cracks. After 

 a time this layer of cork in its turn gets stretched and 

 cracks, and the process is repeated, a new cork cambium is 

 formed behind the old one and makes another layer of 

 cork. 



The structure of bark is thus very complicated. What is 

 ordinarily called bark consists of two parts. All that is out- 

 side of the latest formed cork is dead matter, because no water 

 or food supply can get to it from the interior of the tree. 

 This is the true bark which serves to protect the tree from 

 outside dangers. It may remain for a long period or it may 

 strip off jn patches or rings. All between the cambium and the 

 cork cambium is the inner bark, or bast. This remains alive, 

 and through it the food material passes from the leaves to 

 any part of the tree. 



The structure of wood. All wood is made up of very small 

 tubes and cells. In the latest formed wood the tubes act 

 as pipes through which the water flows up the tree. In this 

 part the wood is usually soft and light in colour, and it is 

 known as the sap- wood. 



In the centre of the tree the pipes are blocked up and are no 

 longer used for the transit of water. They are, in fact, dead, 

 and they usually become hard and tough. This is the heart- 

 wood^ which serves to give strength to the stem. The heart- 

 wood is not necessary for any vital action, and the tree can 



