8 OUTLINE OF STRUCTURAL BOTANY 
Another form of modified stem is the corm (Fig. 9). It is a 
solid, rounded mass at the foot of the stem, often flattened, as it is 
in case of the Jack-in-the-pulpit. It resembles a bulb in form, but 
differs from it in structure. 
The bulb, which occupies a position at the foot of the stem simi- 
lar to that of the corm, consists of a compact mass of scales from 
the midst of which proceeds a stem (Fig. 10). Both the corm and 
the bulb give out roots from their base. 
A tuber is an expansion of an underground stem in which is 
stored nutrient material which may be, after the dying down of the 
aerial part of ‘the plant, supplied to the young buds which have 
been formed on the surface of the tuber. The common white 
potatoe is the best example of such a tuber, its eyes being in fact 
so many buds which sprout in the spring and:are fed by the starchy 
material of the tuber. 
Other forms of modified stems, runners, stolons and suckers are 
less underground forms than those above described and will be 
mentioned in connection with the stem. ; 
THE STEM 
As the root is usually the descending axis of the plant, so the 
stem is ordinarily the ascending axis, though, as we have seen 
above, the modified stem, the rootstock, the tuberous root, etc., are 
buried beneath the soil. 
With exceptions of a few minute species all flowering plants are 
furnished with a stem, although in the case of those improperly 
said to be stemless—acaulescent—the stem is extremely short, con- 
sisting only of a narrow ring above the caudex of the root. 
Thus the stem may vary in extent from the simple ring above the - 
root to the height of a giant sequoia. 
The young stems of most plants bear, at more or less irregular 
intervals, leaves and buds. ‘If we examine the young twig of a 
birch we notice that the distance between the leaves diminishes 
toward the outer extremity until at the end we find a terminal bud 
which is, in fact, a collection of rudimentary leaves often covered 
by scales, themselves modified leaves, at the point at which the 
future growth is to take place. At the axil of each leaf and just 
above we notice a small top-shaped body, a lateral bud. It is 
destined to put forth as a branch at a future time. The future 
branches, of which these lateral buds are the promise, will them- 
