22 TYPES OF BRITISH PLANTS 
outside, and in a year or two we have roughly three 
concentric rings. Outside is a ring, or hollow cylinder 
of bast; next to that is a busy ring of cambium cells; 
and behind this is the solid mass of wood, projecting 
into the pith with which nearly the whole stem once 
was filled. 
Year by year the process goes on, only, now that the 
bast is complete, almost the entire energy of the cambium 
goes to building up the wood with layer on layer of cells. 
All the protoplasm speedily goes from these cells. They 
will grow no more, and only serve to carry water or air 
about the plant or tree, and to give it strength. This 
is why a hollow tree lives so long. The wood that has 
gone, though probably useful, was in no way necessary to 
its growth, and the tree can manage very well without 
it. On the other hand, we can see how it is that trees 
can be destroyed by “girdling,” as it is called, a method 
in use when it seems too much trouble to cut a tree 
down. A cut is made completely round the trunk, not 
very deep nor very wide, but, like Mercutio’s wound, 
twill serve. The dead bark matters nothing, but if the 
thin bast layer and the cambium ring are cut the whole 
of the plant must die. | 
The age of a tree can, as you know, be counted 
accurately from the number of rings that show upon 
the severed trunk, and the reason of this is somewhat 
curious. The apparent ring marks the junction of the 
new wood of spring with that formed in the previous 
autumn; but one may ask why there should be any 
obvious difference. The reason is that the cambium 
ring, which is always building up its wood-cells towards 
the centre of the tree, makes much larger cells in spring 
than in autumn. In winter, of course, the building stops 
