54 ESSENTIALS OF BOTANY 
69. Indefinite Annual Growth.— In most of the forest 
trees, and in the larger shrubs, the wood of the branches 
is matured and fully developed during the summer, and 
protected buds are formed on the twigs to their very tips. 
In other shrubs — for example, in the sumac, the rasp- 
berry, and blackberry — the shoots continue to grow until 
their soft and partly matured tips are killed by the frost. 
Such a mode of growth is called indefinite annual growth, to 
distinguish it from the definite annual growth of most trees. 
70. Trees, Shrubs, and Herbs. — Plants of the largest 
sizé, with a main trunk of a woody structure, are called 
trees. Shrubs differ from trees in their smaller size, and 
generally in their more forking and divided stem. The 
witch-hazel, the dogwoods, and the alders, for instance, 
are most of them classed as shrubs for this reason, though 
in height some of them equal the smaller trees. Some of 
the smallest shrubby plants, like the blueberry, the win- 
tergreen, and the trailing arbutus, are only a few inches 
in height, but are ranked as shrubs because their woody 
stems do not die quite to the ground in winter. 
Herbs are plants whose stems above ground die every 
winter. 
71. Annual, Biennial, and Perennial Plants. — Annual 
plants are those which live but one year, biennials those 
which live nearly or quite two years. 
Some annual plants may be made to live over winter, 
flowering in their second summer. This is true of winter 
wheat and rye among cultivated plants. 
Perennial plants live for a series of years. Many kinds 
of trees last for centuries. The Californian giant red- 
woods, or Sequoias, which reach a height of over 300 feet 
under favorable circumstances, live nearly 2000 years; and 
