STEMS AND BRANCHES AND THEIR USES 221 



it grows and stores away food (in the carrot and beet this 

 is stored in large quantities in the root) ; in the second 

 year it sends up a flower-bearing shoot, forms seeds, and 

 then dies. There are some plants that hve more than 

 two years, but which, like biennials, flower only once in their 

 life and then die. Bamboos belong to this class ; so does the 

 century plant, which is said to live from ten to fifty years 

 before it bears flowers. The plants of another large class live 

 for many years and bear many sets of flowers. These are 

 perennials. 



The long life of a perennial is due to the fact that it con- 

 tinues to grow and produce new tissues as its older parts die. 

 It is not true that any one particular part of the plant lives 

 for many years ; the parts of an old plant that are now alive 

 may be only a small fraction of the tissues that the plant has 

 produced. In an old tree, for example, the heartwood, which 

 makes up a large part of the bulk of trunk, branches, and 

 roots, is dead. So are most of the full-grown cells of the 

 sapwood, as well as all the cells in the outer part of the bark. 

 In the case of a plant with a long underground stem, like the 

 may-apple, the older parts of the stem and of the branches 

 are steadily dying. Most annuals are herbaceous plants. 

 Most perennials are woody, or at least the parts that live 

 for more than a year are woody. 



239. Limits of Life of Perennials. — A plant can live 

 only so long as food substances can be carried between the 

 absorbing root hairs (borne just back of the root tips) and 

 the living tissues of its stem, branches, and leaves. The 

 root tips of a tree or of a climbing plant are continually 

 pushing farther into the soil, and the younger parts of the 

 stem and branches which bear most of the growing buds and 

 leaves are growing farther upward and outward. So, as the 

 plant grows older, water must be carried farther, and it 

 becomes more and more difficult to supply water to the grow- 

 ing parts. In time the supply of water will fall off, and as a 



