In Britain and in most of its Continental habitats two varieties exist, regarded by many as distinct species: one, Q.
In fresh-water Hydromedusae the life-cycle is usually secondarily simplified, but in marine forms the life-cycle may be extremely complicated, and a given species often passes in the course of its history through widely different forms adapted to different habitats and modes of life.
The cotton-wood timber, though soft and perishable, is of value in its prairie habitats, where it is frequently the only available wood either for carpentry or fuel; it has been planted to a considerable extent in some parts of Europe, but in England a form of this species known as P. monilifera is generally preferred from its larger and more rapid growth.
In these northern habitats it attains a large size; the wood is very soft; the buds yield a gum-like balsam, from which the common name is derived; considered valuable as an.
Thus A plant formaiion is a group of associations occupying habitats - iich are in essentials identical with each other.
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