84 ROOTS OF PLANTS. 
has been said by some authors that the roots of trees 
spread as much beneath the ground as the branches 
do above it; but this can hardly be said to hold 
good in all cases. Fibrous-rooted plants often per- 
form great service in loose sandy soils, especially 
along water-courses, where they form a thick and 
matted mass, thus preventing the washing away of 
the earth. Tuberous roots are solid and very irregular 
in their shape, and are often linked together by slen- 
der fibres. Roots of this form are the most useful, as 
they are generally edible; the common potato, the 
turnip, and the radish, are familiar examples. 
Bulbous roots are of various kinds ; some are solid, 
as in the crocus; others are composed of fleshy layers 
placed one above the other, as in the onion ; and others 
consist of thin scales, as in some species of the lily. 
They all appear to act as reservoirs for the vitality 
of the plant during its dormant state. 
A bulb is entirely analogous to the bud upon a 
tree, each containing within itself the embryo of the 
future stem or plant. In the bulb of the tulip, the 
microscope will reveal the entire leaf, stem and flower, 
all folded up within its layers, and which require no- 
thing but the action of light, heat and moisture, to 
expand into perfection ; so in the bud upon the tree, 
the leaves and blossoms which open in the spring are 
all encased in miniature in that tiny compass. 
