HERD. 69 
CHAPTER XI. 
THE STEM, OR ASCENDING AXIS. 
161. Tuat part of the plant which originates with the plum- 
ule (124, 5), and arises above the surface, expanding itself to 
the influence of the air and the light, is called the ascenpine 
AXIS OF STEM. 
a. The cause of its upward tendency is unknown (131, note), but is supposed 
to be in some way due to the principles of light and gravitation. 
162. Although the first direction of the stem’s growth is vertt- 
cal, there are many plants in which it does not continue so, but 
extends in an oblique or horizontal direction, either just above 
the surface of the ground, or just beneath it. When the stem 
continues to arise in its original direction, it is said to be erect. 
When it grows horizontally upon the surface, itis said to be 
procumbent, creeping, trailing, &c. When it arises obliquely it 
is an ascending stem, and when it continues buried beneath the 
soil it is a subterranean stem. 
a. The subterranean stem, and some varieties of the creeping, have usually 
been described as roots. 
163. In regard to duration, the stem, like the root, is said to 
be annual when it lives but one season, afterwards dying, at 
least down to the root, and perennial when its existence is con- 
tinued beyond one season, to an indefinite period of time. 
164. In regard to the size and duration of the stem, plants are 
distinguished into trees, shrubs, and herbs. 
a. A TREE is a plant with a perennial, woody stem, or trunk, 
which does not divide into branches for a certain distance above 
the ground. Ex. elm, palm. 
6. A suRvuB is a plant of smaller dimensions than a tree, hav- 
ing a perennial, woody stem, which divides into branches at or 
near the ground, like the alder. A shrub of diminished size is 
termed an wndershrub. Ex. whortleberry. 
c. An HERB is a plant with an annual or perennial root, pro- 
