THE STEM 103 
omitted by the stem. Many grasses, like the bamboo 
and couch grass, have the rhizome habit and are, for 
that reason, hard to eradicate. Their subterranean 
stems push their way through the soil, but little hin- 
dered by the presence of other plants, and, branching 
from time to time, soon spread over large areas. This 
is why rhizome plants, in the struggle for existence, 
are able to starve out most others. 
The tuber (Fig. 72) is a swollen portion of an 
underground stem, of which the potato is the common 
example. The eyes are the embryo branches and are 
situated at the nodes, lying in the axils of extremely 
small seale-lke leaves. These scale leaves are much 
more distinct in the Jerusalem artichoke, another 
common garden tuber. 
A bulb consists of a short conical stem surrounded 
by the fleshy bases of leaves which contain the 
food reserves. There are two types of bulb, the 
tunicated (Fig. 73), in which the leaves completely 
sheathe the stem, and the scaly bulb (Fig. 74), in 
which the fleshy scale-lke bases do not sheathe, but 
are arranged in a spiral on the conical axis. The 
onion and lily are respectively familiar examples of 
tunicated and sealy bulbs. In the spring, the eonical 
axis lengthens into the flowering stem, and some of 
the leaves appear above ground and become green. 
At first, the chief nutriment is drawn from the fleshy 
leaf bases, but when chlorophyll is developed, carbon 
is taken from the air and used, not only to supply 
the growing shoot, but to form fresh reserves for next 
year’s stem. Young bulbs develop in the axils of the 
fleshy leaves and provide for vegetative reproduction. 
A corm (Fig. 75), well seen in the gladiolus and 
crocus, is a short, fleshy, thickened stem covered by 
“membranous leaves. At the top, alongside the remains 
of last year’s flowering stem, and hidden by the leaf 
scales, is the young shoot which will, in due season, 
