ORGANIC STRUCTURE. 109 



ness of the layers according to the age of the tree ; 

 when very young or very old the layers are less than 

 when the tree is in vigorous youth. 



From the time each individual layer conies into 

 visible existence till the period of its decay, it under- 

 goes some very material changes. When first visi- 

 ble it is called cambium, and next alburnum by 

 botanists ; and by woodmen and carpenters the sap- 

 wood. In this stage it forms the principal channel 

 for the flow of the sap ; but after a certain, or rather 

 an uncertain number of years, it becomes changed in 

 colour and consistence, and henceforth is called per- 

 fect or heart-wood. In some trees, as the beech, the 

 wood soon gains perfection. The oak has generally 

 six or seven, sometimes more, layers of alburnum 

 or sapwood, and all the interior perfect. 



The change appears to be caused by the white 

 wood gradually ceasing to be a channel for the con- 

 duction of the sap ; and to a certain chemical action by 

 which its colour and texture is changed from a white 

 and soft, to a brown and harder consistence. The sap 

 also, which becomes secreted in the intercellular spaces 

 and vessels of the mature wood, assumes a concreted 

 form and acts as a cement to the fibrous structure. 



As different kinds of trees have wood of very dif- 

 ferent degrees of hardness and durability, the follow- 

 ing questions occur ; does the durability depend on 

 the texture or density of the ligneous structure, or 

 on the quality of the concreted juices therein 



