I have said that the legitimate use of poplars in ornamental grounds is
in the production of minor or secondary effects. As a rule, they are
less adapted to isolated planting as specimen trees than to using in
composition,--that is, as parts of general groups of trees, where their
characters serve to break the monotony of heavier forms and heavier
foliage. The poplars are gay trees, as a rule, especially those, like
the aspens, that have a trembling foliage. Their leaves are bright and
the tree-tops are thin. The common aspen or "popple," _Populus
tremuloides,_ of our woods, is a meritorious little tree for certain
effects. Its dangling catkins (Fig. 33), light, dancing foliage, and
silver-gray limbs, are always cheering, and its autumn color is one of
the purest golden-yellows of our landscape. It is good to see a tree of
it standing out in front of a group of maples or evergreens.

[Illustration: Fig. 34. Plant-form in cherries.--Reine Hortense.]

Plant-forms.

Before one attains to great sensitiveness in the appreciation of
gardens, he learns to distinguish plants by their forms. This is
particularly true for trees and shrubs. Each species has its own
"expression," which is determined by the size that is natural to it,
mode of branching, form of top, twig characters, bark characters,
foliage characters, and to some extent its flower and fruit characters.
It is a useful practice for one to train his eye by learning the
difference in expression of the trees of different varieties of cherries
or pears or apples or other fruits, if he has access to a plantation of
them. The differences in cherries and pears are very marked (Figs.
34-36). He may also contrast and compare carefully the kinds of any
tree or shrub of which there are two or three species in the