When the generation is of only one season’s duration, the plant is said to be =annual=. When it is of two seasons, it is =biennial=. Biennials usually bloom the second year. When of three or more seasons, the plant is =perennial=. Examples of annuals are pigweed, bean, pea, garden sunflower; of biennials, evening primrose, mullein, teasel; of perennials, dock, most meadow grasses, cat-tail, and all shrubs and trees. =Duration of the Plant Body.=--Plant structures which are more or less soft and which die at the close of the season are said to be =herbaceous=, in contradistinction to being =ligneous= or =woody=. A plant which is herbaceous to the ground is called an =herb=; but an herb may have a woody or perennial root, in which case it is called an =herbaceous perennial=. Annual plants are classed as herbs. Examples of herbaceous perennials are buttercups, bleeding heart, violet, water lily, Bermuda grass, horse-radish, dock, dandelion, golden rod, asparagus, rhubarb, many wild sunflowers (Figs. 11, 12). Many herbaceous perennials have _short generations_. They become weak with one or two seasons of flowering and gradually die out. Thus, red clover usually begins to fail after the second year. Gardeners know that the best bloom of hollyhock, larkspur, pink, and many other plants, is secured when the plants are only two or three years old. Herbaceous perennials which die away each season to bulbs or tubers, are sometimes called =pseud-annuals= (that is, _false annuals_). Of such are lily, crocus, onion, potato, bull nettle, and false indigo of the Southern states. True annuals reach old age the first year. Plants which are normally perennial _may become annual in a shorter-season climate by being killed by frost_, rather than by dying naturally at the end of a season of growth. They are climatic annuals. Such plants are called =plur-annuals= in the short-season region. Many tropical perennials are plur-annuals when grown in the north, but they are treated as true annuals because they ripen sufficient of their crop the same season in which the seeds are sown to make them worth cultivating, as tomato, red pepper, castor bean, cotton. Name several vegetables that are planted in gardens with the expectation that they will bear till frost comes. [Illustration: FIG. 13.--A SHRUB OR BUSH. Dogwood osier.] Woody or ligneous plants are usually longer lived than herbs. Those that remain low and produce several or many similar shoots from the base are called =shrubs=, as lilac, rose, elder, osier (Fig. 13). Low and thick shrubs are =bushes=. Plants that produce one main trunk and a more or less elevated head are =trees= (Fig. 14). All shrubs and trees are perennial. [Illustration: FIG. 14.--A TREE. The weeping birch.] Every plant makes an effort _to propagate, or to perpetuate its kind_; and, as far as we can see, this is the end for which the plant itself lives. _The seed or spore is the final product of the plant._