Trees—The Character, Structure, Sec. 43 



wood everywhere on its inner surface, but, to a smaller extent, 

 living bark {hast) on its onter surface ; the latter conveys the 

 manufactured food from the leaves to the cambium, for it is 

 important that it sliould be propei'ly nourished if new zones of 

 wood arc to be produced. It is very sensitive to external 

 injuries, and is protected by a jacket of corky tissue — the bark, 

 hence mechanical injuries such as scraping and cutting bark by 

 boys is one to be deprecated, as it reduces the protection by 

 exposing the living parts of the trunk to insects and micro- 

 organisms which cause disease. The zones of wood are repeated 

 each year, in most trees, in a very definite a.nd characteristic 

 manner according to the species of tree. 



Rings. — Having got some idea of the tissue responsible for 

 the production of wood, let us now consider the rings of wood 

 produced annually by the division of the Cambium. These rings, 

 in many woods, are clearly marked because of the wood formed 

 early in the year. Springwood as it is usually called, and which 

 lies nearer the pith in the ring, is of a softer and looser nature, 

 and therefore lighter in colour than the wood formed later in the 

 season, and usually known as late or summer wood. 



Why is there this contrast in tissue elements at different 

 seasons of the year in many European Avoods 1 To some extent 

 it must be due to tension caused by the tight bark compressing 

 upon the increasing growth of wood, which, in its turn, is stretched 

 and strained by the latter, as is proved by the longitudinal 

 fissures which appear in the bark of many trees. Smooth bark 

 is taut and presses upon the increasing girth, and if a longitudinal 

 incision is made in such bai^k it will gape, while, if a strip of bark 

 is cut ofT and then replaced immediately before it dries, it will 

 be found impossible to make the edges meet. This means that 

 bark presses on the growing wood, like a tightened iron band, 

 which increases until the autumn is reached. This increased 

 pressure causes the later formed wood elements to become flat 

 and thicker walled. If an artificial local pressure is set up in 

 early spring, by means of an iron band, and allowed to remain 



