ROOTS AND UNDERGROUND STEMS. 87 
tage of by the agriculturist and gardener, and by their labours 
in developing what is already a natural tendency of these 
plants we have acquired many varieties of turnips, carrots, 
parsnips, beetroot, and radish, which serve as food for man 
and beast. Were these plants to be left to grow for a second 
year, they would use up, in the production of their seed, all 
the material stored during their first year. 
It has been already said that in all the plants hitherto 
considered the first root is a tap-root, developing directly 
from the elongation of the radicle of the embryo. But, besides 
these, roots may arise in other ways, and all such are included 
in the general term adventitious roots. For example, when 
we put a cutting, say of a geranium, into the ground, it 
usually puts out such adventitious roots, and develops into a 
new plant. Many plants do this naturally (e.9., strawberry), 
and it is one of thei: normal means of reproduction. And, 
further, in the tropics and in humid regions aerial roots are 
common. These either remain permanently above the ground, 
as in epiphytes (see p. 94), or are sent down from the branches 
to the soil or mud below, as in the Banyan tree of India. 
Lastly, a few plants develop special forms of adventitious 
roots for the purpose of support, as in the Ivy; but these do 
not serve at all to nourish 
the plant. 
Underground stems 
are readily distinguished 
from roots by them pyro- 
ducing buds, from which 
leaves and leaf - bearing 
branches are sent up. They 
may be classed under three 
heads —viz., rhizomes, 
tubers, and bulbs — but 
popularly all are known as 
roots. 
The rhizome,* or root- 
stock, is always elongated, 
and sends roots down from 
its lower, and leaves and 
branches up from its upper, 
surface. Rhizomes are vari- 
ous in form and appearance. 
Usually they are somewhat 
thickened, and creep along 
either at or just below the 
surface of the soil. Fre- Fig, 172. Rhizome of Iris. 
* Gr. rhiza, a root, from their resemblance to roots. 
