Botanists have divided roots into three classes, according to their shape or figure, their situation in the ground, and their duration.
I. With respect to figure, roots are either simple ; spindle-shaped, as in the carrot ; bitten off, as in the devil's-bit scabious; bulbous (see Bulb, vol. i. p. 386) ; tuberous, as in the potatoe; bead-ed; blanched , hair-like ; jointed; scaly ; pendant ; toothed ; fascicu-lated, or bundled.
II.The roots, which are denominated from their situation, are either perpendicular ; horizontal ; oblique; creeping; zig-zag, or in-flected; or such as put forth suckers. III. With regard to their duration, roots are either annual, that is, they flower and decay in one year; biennial, when they continue to vegetate two years; and perennial roots, are such as flourish for several, or at least more than two, years.
These essential parts of plants greatly contribute to the comfort, and to the benefit of mankind ; as many of them not only afford whole-some and nutritious food, but are of considerable utility in medicine. Several roots also impart colours, which are employed both in arts and in manufadtures; and are, in general, more durable than those obtained from the plants. Thus, the expressed juice of the common radish, when combined with to-bacco-pipe clay and a little alum, yields a blue of considerable per-manency and brightness.
Notwithstanding their tinging properties, however, the generality of roots is etiolated, or perfectly white, in consequence of their seclusion from the light. This phenomenon, in the opinion of Dr. Darwin, arises from the liberation or evolution of their superfluous oxygen, which unites with the colouring matter, and converts the latter into a colourless acid ; excepting in such roots as contain too large a proportion of the dye-ing substance, for instance, in the madder; the roots of which, externally, are red, while the internal part is yellow.—See also Light.