180 LIFE OF A TREE. 
made, not ‘as in Exogens to the outside, but 
to the interior of the stem, and hence called 
Endogens, or growers from within,—Exogens 
being growers from without,—in Endogens then, 
it is easy to imagine that a limit to their growth 
may exist. Let us explain this matter in the 
following homely way. Suppose we had a hollow 
wooden cylinder before us, such as a_ barrel 
with the ends knocked out. Now, plainly, we 
might put as many large barrels as we chose 
over and outside the first, until we could only 
put no more because no one could make us a 
barrel large enough to fit over the last. But 
suppose we tried to cram the barrel full of rods 
or poles, we should succeed in getting a certain 
number in, but with all our efforts we could 
not get more, simply because the barrel would 
not stretch out larger. ‘This circumstance then 
alone would limit us in our attempts to add to 
the size of the barrel; while it 1s manifest in the 
other case that there is no limit to its increase in 
size, but our inability to obtain a larger outside 
barrel. Had we the power to make barrels of 
any size, we might go on fitting one over another, 
until the great tun of Heidelberg, about which 
