STEMS. 47 
In the simplest form of rootstock, Fig. 33, such as is found 
in some mints and in many grasses and sedges, the real 
nature of the creeping stem is shown by the presence upon 
its surface of many scales which are reduced leaves. In the 
stouter rootstocks, like that of the iris, Fig. 54, this stem-lke 
character is less evident. The potato is an excellent example 
of the short and much thickened underground stem known 
as a tuber. 
It may be seen from Fig. 35 that the potatoes are none of them 
borne on true roots, h, but only 
on subterranean branches, e, e, 
which spring from buds formed 
in the axils of the cotyledons. 
Fic. 31.— Coiling of Petiole of Dwarf 
Tropxolum ; /, leaf; a, petiole. Fig. 32.— Twining Stem of Hop. 
Bulbs, whether coated like those of the onion or scaly like 
those of the hyacinth, Fig. 36, are merely very short and 
stout underground stems, covered with closely crowded scales 
or layers which represent leaves or the bases of leaves, Fig. 37. 
The variously modified forms of underground stem just 
discussed, illustrate in a marked way the storage of nourish- 
