CLASSES OF STEMS 171 
ledonous, according to whether or not they belong to Monocoty- 
ledons or Dicotyledons. However, it is more in structure than 
in external characters that these two types of stems present 
important differences. 
As to hardness stems are often classified as either herbaceous or 
woody. Stems that are typically herbaceous, like those of Clover, 
Alfalfa, Tomatoes, and others which develop very little woody 
tissue, are soft and short-lived, usually living but one year. It is 
among trees, where the amount of woody tissue reaches its maxi- 
mum, that the best examples of woody stems occur. However, 
between herbaceous and woody stems there is no distinct line of 
division, for most herbaceous stems are woody in their older 
regions and all woody stems are herbaceous in their younger 
regions. The terms, herbaceous and woody, refer, therefore, to 
the amount of woody tissue present, and not to the presence or 
absence of it. 
As to length of life stems may be classified into annuals, bien- 
nials, and perennials. Annual stems live but one growing season. 
The stems of most herbaceous plants are annuals, dying down to 
the ground either before or after frost comes, as in case of most 
vegetables, weeds, and Grasses. But annual stems and annual 
plants must not be confused, for many plants, like Alfalfa, Quack 
Grass, and Canada Thistle, which live many years, thus being 
perennial in habit, have annual stems which grow up in the spring 
and die down in the fall. When the plant is annual, roots, stem, 
and all other parts die at the end of the growing season, and 
the plant must be started anew from seed. 
In plants, such as Turnips, Carrots, and Beets, which require 
two years to complete their life cycle, and are, therefore, known 
as biennials, the stem remains short during the first growing season, 
forming a mere crown at the top of the root. During the second 
growing season, stems develop which bear flowers and seeds, 
and then the entire plant dies. In some biennials, as Cabbage 
and Rape illustrate, the stem is prominent during the first season, 
although it elongates much more during the second season in 
preparation for bearing flowers and seeds, as shown in Figure 148. 
In Red Clover, Sweet Clover, and many weeds with the biennial 
habit, the portion of the stem known as the crown is biennial, 
while the branches arising from the crown are annuals. 
Perennial stems, so described because they live year after 
