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 living 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 alive, 7¢., 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 inother 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. 
1 This 1s not entirely true, for certain tissues like the pith rays may be 
still considered as lving 
