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 

 size, 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. ]\Iany 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 



