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, hke 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 Hfe 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 I48. 

 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 



