88 PLANT LIFE 
The lateral branches, which in isolated trees 
spread out and constitute its charm, here 
compete with each other, and all are over- 
shadowed by the topmost branches which 
alone get properly illuminated. Consequently 
the lower leaves become useless, and they, 
together with the branches which bear them, 
become starved and are destined to perish. 
The tops are constantly growing higher, while 
the trunk is as perpetually being denuded 
of its lower lateral branches. In this way 
the grand boles or trunks are formed which the 
woodman delights to see, and they are the 
distinguishing features of forests managed with 
skill and intelligence. The forester’s aim is 
always secured, broadly speaking, in this 
way, though of course there are differences 
in actual treatment depending on the par- 
ticular kind of tree or association of trees 
it is desired to produce. The tall trunks are 
the result of a sort of natural pruning, brought 
about by growing the trees at the correct 
distances apart, the actual distance being 
regulated by the size, age, and other conditions 
which affect the growth as a whole. 
The amount of leaf surface is not only 
influenced by the available exposure to light, 
but is influenced by other conditions as well. 
Thus mechanical requirements need to be 
satisfied, and they may easily limit the dimen- 
sions practically attainable by the green 
surface as a whole. A leaf too weak in itself, 
or too feebly supported to retain a suitable 
