78 GENERAL BOTANY 



they can be readily separated when in active growth 

 because between them is a very thin layer of delicate 

 tissue which is easily ruptured. 



This layer is called the cambium, an'd by its growth 

 and development woody tissue is added to the core, 

 and soft or stringy fibrous- tissue to the bark. So 

 that the core grows centrifugally by the addition of 

 new layers of wood to the outside, and the bark 

 centripetally by new layers on its inner side. 



Now it is only through small tubes in the young- 

 est parts of the wood and the bark that sap passes up 

 from the roots and down from the leaves, the middle 

 of the core being useless for this purpose. 



At the beginning of the hot weather, when the 

 buds are opening out and growth is rapid, the new 

 tubes of wood that are formed are larger than later 

 on in the year when growth is slower ; and by the 

 regular alternation of the wet and dry seasons year 

 after year, there are formed alternating zones of 

 more and of less compact and hard tissue. This 

 is why, when the trunk or branch of a tree is cut 

 across, a number of concentric rings can generally 

 be seen. In countries where the difference between 

 the seasons is more marked thari in the tropics, these 

 rings are always very distinct and show by their 

 number the exact age of the part ; for each ring 

 corresponds to a winter or a summer, and indicates 

 one year's growth. 



This is also the cause of the * grain ' of timber 

 those lines and markings which can be seen on nearly 

 all wood. By feeling with a knife one can at once 

 tell which are the hard winter or dry weather and late 



