1350. Water is absolutely necessary to the economy of vegetation in its elastic and fluid states; and it is not devoid of use even in its solid form. Snow and ice are bad conductors of heat; and when the ground is covered with snow, or the surface of the soil or of water is frozen, the roots or bulbs of the plants beneath are protected by the congealed water from the influence of the atmosphere, the temperature of which, in northern winters, is usually very much below the freezing point; and this water becomes the first nourishment of the plant in early spring. The expansion of water during its congelation, at which time its volume increases one-twelfth, and its contraction of bulk during a thaw, tend to pulverise the soil, to separate its parts from each other, and to make it more permeable to the influence of the air.
