FS 
66 THE ROOT. 
from the joints of creeping or prostrate plants; as the. ground- 
ivy, and the twin-flower (Linnea). 2d, The roots of certain 
erect plants of the endogenous structure, originating from the 
stem high in air, descending and entering the soil. Of this class 
the screw-pine (Pandanus) is a remarkable example, whose 
aerial roots are often several feet in length before reaching the 
earth. Such roots, a few inches in length, are also seen in the 
common maize (Zea). 
b. A third class of aerial roots is peculiar to the epiphytes (et, 
upon, putov,a plant). These plants are fixed upon the trunk 
and branches of other species, and derive their nourishment 
chiefly from the air. Such are the long moss (Tillandsia), pen- 
dent from lofty trees, and many of the Orchidacez at the south. 
4th, The roots of parasites are usually aerial. These are not 
only attached to other vegetables, but, penetrating their tissues, 
they derive nourishment from their juices. The Cuseuta and 
Mistletoe are examples. 
PHYSICAL STRUCTURE AND FUNCTIONS OF THE ROOT 
151. The internal structure of the root is similar to that of the 
stem (q. v.), except that there is often a greater proportion of 
cellular, fleshy matter, as in the beet. In Endogens the root 
is endogenous, in Exogens it is exogenous, but in the latter case 
it is always destitute of a pith. 
152. The fibrils are in fact but subdivisions of the caudex, or 
main root. They consist of minute bundles of vasiform tissue 
(32), enclosed in a loose, cellular epidermis, except at the ex- 
tremities (85), where the tissue is naked and becomes exceed- 
ingly loose and spongy. These (spongioles) have the property 
of powerfully absorbing water. 
153. The growth of the root does not take place by the ex- 
pansion of the parts already formed, but simply by the addition 
of new matter at the extremities, and by the formation of new 
layers upon the surface. This accounts for the facility with 
which it penetrates the crevices of the soil, and forces its way 
into the hardest earth. 
154. The most obvious function of the root is the purely 
mechanical one of fixing the plant in the earth, and maintaining 
