THE CONICAL GKOWTH OF TEEES. 65 



in the fibre cells after the first year, is the gradual filling up of 

 their cavities by deposits of lignine or earthy matter, whilst the 

 duct cells, being devoid of sap, after the first year are never 

 filled up, as inspection plainly shows. 



Let the reader again refer to the diagram, which shows, not 

 only the conical growth of the primary axis of the branch, but 

 also the conical growths of its secondary and tertiary axes, or 

 branchlets and twigs, and how they are connected with the pri- 

 mary axis. Commencing with the first, second, or third ring 

 from the outside, which form the foundation of three successive 

 conical deposits,' and tracing them upward over the summits of 

 each branch and branchlet, he will find that each forms a con- 

 tinuous and unbroken bed of woody matter ; and he will easily 

 understand, when the tree grows again in Spring, (if we suppose 

 the thick exterior and bounding line of the diagram to represent 

 the newly-forming stratum of bark and wood cells,) how, from 

 different points of the surface newly-forming on the last year's 

 branches and branchlets, fresh shoots may be pushed forth. It 

 is thus, as the tree continues to grow, that a series of new and 

 more extensive communications are annually opened with that 

 grand reservoir of vegetable food, the atmosphere; and the 

 newly -deposited living stratum is just as continuous as that of 

 former years, and its life passes away with the falling of the 

 leaves. For it is the leaves which are the source of the forma- 

 tive material which proceeds from them to the shoots, from the 

 shoots to the branchlets, and from the branchlets to the branches 

 whose union forms the main stem of the tree, just as a thousand 

 little streamlets, descending from mountain and hill, pour 

 together their tributary waters, which, united, form the broad 

 river which rolls on to the ocean. 



But, although the number of leaves of the whole tree, as of 

 each branch, from year to year continues to increase, yet the 

 thickness of the conical layers, after a certain point of time in 

 the development of the main axis or stem, as regularly dimin- 

 ishes. The cause of this decrease in the breadth of the wood- 

 rings arises from the fact, that the surface of the enveloping 

 cones continually increases as the tree gets older, so that the 

 formative material, from the leaves, is spread over a more and 

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