ILLINOIS NATURAL HISTORY SURVEY MANUAL 1 
cells at the tip, called a root cap, and discharging three principal 
functions—anchorage, absorption of water and certain mineral 
salts, and food storage. 
They may be grouped into two common systems—faproot 
and diffuse. The taproot system is like that of the parsnip, 
with a main central root extending vertically into the soil 
and having many much smaller branches. In the diffuse root 
system there are numerous roots of approximately equal size that 
extend downward and outward in all directions. The majority 
of diffuse-rooted plants have very fine or fibrous roots, as corn 
or other grasses, but some like asparagus and rhubarb have 
many thick roots of diffuse arrangement. Roots of most flowering 
plants are in the ground but in some instances they are aerial 
or in water, and it is to be remembered that not all underground 
structures are roots. 
Stems.—That portion of the plant that bears the leaves 
is the stem, and that place on the stem where the leaves are 
normally borne is called a mode. Also at the nodes, branch stems 
are developed from the sides of the main stem. There are many 
modifications of stems which at times are not easy to recognize; 
of them the following will be met in the descriptions of plants 
in this book: rhizome, tuber, bulb, corm, stolon, tendril and 
scape. 
MODIFIED FORMS OF STEMS 
The rhizome or rootstock is an underground stem which ts 
usually elongated and horizontal. It grows from a terminal bud 
and produces lateral buds at its nodes. A curious thing about 
the rootstocks of perennials is that all of them in the same species 
are found at approximately the same distance beneath the sur- 
face of the soil. If in growing through the soil these underground 
stems encounter a depression, they grow downward; if they . 
encounter an elevation they grow upward, so as to maintain 
their usual distance beneath the surface. This phenomenon, 
which is not well understood, is sometimes called the “law of 
level.” 
A fleshy underground stem, much enlarged and short, 
usually ovoid or oblong, and also bearing a terminal and several 
lateral buds, is called a tuber. The common potato is our best 
example. 
A bulb is a short underground stem, tipped by a bud, from 
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