forest-trees are leafless; and if we visit an arboretum in the latter part of October, we may select the American from the foreign species, by observing that the latter are still green, while the others are either entirely denuded, or in that colored array which immediately precedes the fall of the leaf. The exotics may likewise be distinguished in the spring by their precocity,--their leaves being out a week or ten days earlier than the leaves of our trees. Hence, if we take both the spring and autumn into the account, the foreign, or rather the European species, show a period of verdure of three or four weeks' greater duration than the American species. Many of the former, like the Weeping Willow, do not lose their verdure, nor shed their leaves, until the first wintry blasts of November freeze them upon their branches and roll them into a crisp.
In a natural forest there is a very small proportion of perfectly formed trees; and these occur only in such places as permit some individuals to stand isolate