130 TAP-ROOT.—AERIAL ROOTS. 
Adventitious roots generally, like true roots, have no leaves 
or buds, and when subterranean have no epidermis furnished 
with stomata; hence when derived from Dicotyledons, they are 
distinguished from the stem by the same characters as that of 
the true root. The adventitious roots of Monocotyledons and 
of Cormophytes have a similar structure to their respective 
stems, as will be afterwards noticed. Aerial roots are, however, 
from their exceptional position, frequently furnished with a true 
epidermis and stomata, and are sometimes of a green colour; 
but in other respects they resemble ordinary adventitious roots. 
The true or primary root, from its being formed by direct 
elongation from the radicle, by additions made within its 
extremity, generally continues to grow downwards for some 
Fie. 254. 
Fig. 254, The Banyan-tree (Ficus indica). 
time at least, and hence forms a main trunk or axis from which 
the branches are given off (fig. 253, 7). Such a root is termed 
a tap-root, and may be commonly observed in Dicotyledons. 
On the contrary, the roots of Monocotyledons and Cormophytes, 
which are adventitious, are usually of nearly equal size, and 
given off in variable numbers from the radicle (fig. 251, 7). 
Some adventitious roots, such as those called aerial, require a 
more particular notice. 
Aerial Roots.—The simplest forms of aerial roots are seen in 
the Ivy (jig. 225, a, a), and some other climbing plants. In 
these plants they are essentially intended for mechanical support, 
and not to obtain food : this they obtain by their ordinary roots 
fixed in the soil. It is probable, however, that in the Ivy and 
other climbing plants some food may be taken up by these roots. 
In many other plants the aerial roots which are given off by the 
