We live in a latitude and upon a soil, therefore, which are favorable to the harmonious grouping of vegetation.  As we proceed southward, we witness a constant increase of the number of species gathered together in a single group.  Nature is more addicted at the North to the habit of classifying her productions and of assembling them in uniform phalanxes.  The painter, on this account, finds more to interest the eye and to employ his pencil in the picturesque regions of frost and snow; while the botanist finds more to exercise his observation in the crowded variety that marks the region of perpetual summer.

But while vegetation is more generally social in high latitudes, several families of Northern trees are entirely wanting in this quality.  Seldom is a forest composed chiefly of Elms, Locusts, or Willows.  Oaks and Birches are associated in forests, Elms in groves, and Willows in small groups following the courses of streams.  Those Northern trees which are most eminently social, including the two just named, are the Beech, the Maple, the Hickory, the coniferous trees, and some others; and by the predominance of any one kind the character of the soil may be partially determined.  There is no tree that grows so abundantly in miry land, both North and South upon this continent, as the Red Maple.  It occupies immense tracts of morass in the Middle States, and is the last tree which is found in swamps, according to Michaux, as the Birch is the last we meet in ascending mountains.  The Sugar-Maple is confined mostly to the Northeastern parts of the continent.  Poplars are not generally associated exclusively in forests; but at the point where the Ohio and the Mississippi mingle their waters are grand forests of Deltoid Poplars, that stamp upon the features of that region a very peculiar physiognomy.

The characteristics of different woods, composed chiefly of one family of trees, would make an interesting study; but it would be tiresome to enter minutely into their details.  Some are distinguished by a superfluity, others by a deficiency of undergrowth.  In general, Pine and Fir woods are of the latter description, differing in this respect from deciduous woods.  These differences are most apparent in large assemblages of wood, which have a flora as well as a fauna of their own.  The same shrubs and herbaceous plants, for example, are not common to Oak and to Pine woods.  There is a difference also in the cleanness and beauty of their stems.  The gnarled habit of the Oak is conspicuous even in the most crowded forest, and coniferous woods are apt to be disfigured by dead branches projecting from the bole.  The Birch, the Poplar, and the Beech are remarkable for the straightness, evenness, and beauty of their shafts, when assembled in a dense wood.