Ecological Points 25 
soils, we mean that they can still prosper under less favor- 
able conditions, if necessary; yet they would do best on the 
soil as described, which is, therefore, the nurseryman’s 
choice. 
As regards light supply to the foliage, we have, of course, 
no means of increasing or modifying it in general, yet in 
eroup planting, park planting, street planting, and even in 
pruning, the knowledge that different species require differ- 
ent light intensity for their best development is essential; 
for here proper adaptation to light conditions, and artificial 
changes in light conditions can be practised. All trees 
thrive ultimately best in full enjoyment of light and then 
develop their characteristic form, as we see it in single trees 
on the lawn; but, if placed in close neighborhood of each 
other, the fact will soon appear that one species is “stronger”’ 
than another; that means the one spreads its branches and 
kills out those of its neighbor by its shade. 
In other words, the foliage of one species requires more 
light to perform its work than that of another. This fact 
is exhibited in the individual tree itself. The interior of 
the crown in those which need much light, like the birch or 
aspen, thins out, is empty of foliage, because of the shade 
which the exterior exercises; it loses its lower branches 
sooner on account of the shading by the upper, or else the 
lower reach out farther to escape suppression. On the 
other hand, the tolerant or shade-enduring kinds, like beech 
and sugar maple, are characterized by compact and dense 
crowns: their foliage even in the dark interior of the crown 
can perform its office. And so a relative scale can be made 
with reference to light requirements from the most light- 
needing to the most shade-enduring. Birch, aspen, willow, 
poplar, and pine are examples of those more sensitive to 
the withdrawal of light; beech, sugar maple, holly, spruce, 
