Cell Structure 7 



that a tree may have its heart riddled and eaten out by- 

 fungi without losing in anything but firmness and stability. 



Cell Structure. The Uving tissue of the bole lies on the 

 outside of the wood, between bark and wood, — a narrow 

 layer of a few cells, called the cambium, enveloping the dead 

 wood. This layer, by division and growth of the cells 

 forming it, makes the new wood of the year, the "annual 

 ring," which again dies for the most part, soon after it is 

 formed, only the outermost cell tissues, the cambium cells, 

 remain fully aUve, i.e., capable of growth and subdivision.' 



Injury to this portion is, therefore, directly of consequence 

 to the welfare of the tree. 



Besides the cambium layer, there are two other points at 

 which persistently living cells are concentrated; namely, 

 the tips of the roots or fibrils, and the tips of the shoots, 

 the so-called growing points or buds. From the buds the 

 shoots and leaves develop, the latter remaining living for 

 only a few months, or, in the case of the needles of conif- 

 erous trees, for a few years. 



It is, then, after all only one year's product that really 

 lives, in the full sense of the word, and this living por- 

 tion encloses a mass of tissues which have lost their life, 

 although they may be still of service to the tree in conducting 

 and storing water or food, in giving stability, or in other ways. 



The outer bark also dies, new bark being formed on the 

 inside next the cambium; and, as the growth of the annual 

 ring of wood and bark on the inside proceeds, the outer 

 dead portions of bark must give way under the pressure of 

 the interior growth. In most cases these dead portions of bark 

 break in characteristic form into fissures, ridges, plates, or 

 scales, which may sooner or later loosen and be sloughed off. 



' This is not entirely true, for certain tissues like the pith rays may be 

 still considered as living. 



