154 ANATOMY OF THE STEM [LESSON 25. 



and often does so, -without the least injury to the tree, except by im- 

 pairing the strength of the trunk, and so rendering it more liable to 

 be overthrown. 



434. The Living Parts Of a Tree, of the exogenous kind, are only 

 these : first, the rootlets at one extremity ; second, the buds and 

 leaves of the season at the other ; and third, a zone consisting of 

 the newest wood and the newest bark, connecting the rootlets with 

 the buds or leaves, however widely separated these may be, in 

 the largest trees from two to four hundred feet apart. And these 

 parts of the tree are all renewed every year. No wonder, there- 

 fore, that trees may live so long, since they annually reproduce 

 everything that is essential to their life and growth, and since only 

 a very small part of their bulk is alive at once. The tree sur- 

 vives, but nothing now living has existed long. In it, as elsewhere, 

 life is a transitory thing, ever abandoning the old, and displaying 

 itself afresh in the new. 



435. Cambium-Layer, The new growth in the stem, by which it 

 increases in diameter year after year, is confined to a narrow line 

 between the wood and the inner bark. Cambium is the old name 

 for the mucilage which is so abundant between the bark and the 

 wood in spring. It was supposed to be poured out there, and that 

 the bark really separated from the wood at this time. This is not 

 the case. The newest bark and wood are still united by a delicate 

 tissue of young and forming cells, called the Camliibm-layer, 

 loaded with a rich mucilaginous sap, and so tender that in spring 

 the bark may be raised from the wood by the slightest force. 

 Here, nourished by this rich mucilage, new cells are rapidly form- 

 ing by division (387-390) ; the inner ones are added to the wood, 

 and the outer to the bark, so producing the annual layers of the 

 two, which are ever renewing the life of the trunk. 



436. At the same time new rootlets, growing in a similar way, are 

 extending the roots beneath ; and new shoots, charged with new buds, 

 annually develop fresh crops of leaves in the air above. Only, 

 while the additions to the wood and bark remain as a permanent 

 portion of the tree, or until destroyed by decay, the foliage is tem- 

 porary, the crop of leaves being annually thrown off after they have 

 served their purpose. 



437. Structure Of the Leaf, Leaves also consist both of a woody 

 and a cellular part (135). The woody part is the framework of ribs 

 and veins, which have already been described in full (13G-147). 



