INTERNAL STRUCTURE. 83 



153. The Living Parts of a tree or shrub, of the exogenous 

 kind, are obviously only these: 1st, The summit of the stem 

 and branches, with the buds which continue them upwards and 

 annually develop the foliage. 2d, The fresh roots and rootlets 

 annually developed at the opposite extremity. 3d. The newest 

 strata of wood and bark, and especially the interposed cambium- 

 layer, which, annually renewed, maintain a living communication 

 between the rootlets on the one hand and the buds and foliage 

 on the other, however distant they at length may be. These are 

 all that is concerned in the life and growth of the tree ; and these 

 are annually renewed. The branches of each year's growth are, 

 therefore, kept in fresh communication, by means of the newer 

 layers of wood, with the fresh rootlets, which are alone active in 

 absorbing the crude food of the plant from the soil. The fluid 

 they absorb is thus conveyed directly to the branches of the sea- 

 son, which develop leaves to digest it. And the sap they receive, 

 having been elaborated and converted into organic nourishing 

 matter, is partly expended in the upward growth of new branches, 

 and partly in the formation of a new layer of wood, reaching 

 from the highest leaves to the remotest rootlets. 



154. Longevity of trees. As the exogenous tree, therefore, 

 annually renews its buds and leaves, its wood, bark, and roots, 

 every thing, indeed, that is concerned in its life and growth, 

 there seems to be no necessary cause, inherent in the tree itself, 

 why it may not h've indefinitely. Some trees are known to have 

 lived for one and two thousand years, and some are possibly 

 older. 1 Equally long may survive such endogenous trees as the 

 Dragon tree (Dracaena) , which have provision for indefinite in- 

 crease in diameter (138), and for the production of branches. 

 The famous Dragon tree of Orotava, in Tenerifie, now destroyed 

 by hurricanes and other accidents, had probably reached the age 

 of more than two thousand years. 



155. On the other hand, increase in height, spread of branches 

 and length of root, and extension of the surface over which the 

 annual layer is spread, are attended with inevitable disadvantage, 

 which must in time terminate the existence of the tree in a way 

 quite analogous to the death of aged individual animals, which 

 is not directly from old age, but from casualties or attacks to 



1 The subject of the longevity of trees has been discussed by DeCandolle, 

 in the " Bibliotheque Universelle " of Geneva, for May, 1831, and in the second 

 volume of his " Physiologie Vege'tale ; " more recently, by Alphonse DeCan- 

 dolle in the " Bibliotheque Universelle ; " and in this country by myself in the 

 " North American Review," for July, 1844. For an account of the huge Red- 

 woods (Sequoias) of California, see Whitney's Yosemite Book. 



