GROWTH OF TREES. 145 



M. A. Richard adopts similar views. He says, if a 

 young branch be examined during the period of vege- 

 tation, that is, when the juices abound in all parts of 

 the plant, there may be observed between the inner 

 bark and the pulp wood a layer of fluid, at first clear 

 and limpid, but gradually becoming thicker and less 

 transparent. This fluid is composed of the descending 

 pulp, and in proportion as it thickens, fibres are seen 

 to form therein , and it soon becomes organised and 

 acquires the appearance of vegetable tissue, a change 

 which is gradual, and continues so long as the buds 

 are growing, so that the formation of the annual layer 

 proceeds in a slow and progressive manner. It is on 

 this account, that the new layers of pulp wood so fre- 

 quently exhibit concentric zones, which shows that 

 their whole thickness has not been formed at once. It 

 hence appears, that the pulp wood is not formed by the 

 inner bark thickening and becoming more consistent, 

 but by the pulp which is organised, and thus becomes 

 the means of growth in diameter, giving rise annually 

 to the formation of a layer of pulp wood, and of a layer 

 of inner bark, both distinct from each other, although 

 produced by the same organ. When Du Hamel found 

 in the pulp wood the silver wire, which he thought 

 had passed through the inner bark, it was because it 

 had been really engrossed in the organised layer of the 

 pulp. It follows also, that the inner bark every year 

 grows thick by its inner surface ; in fact, the layer of 

 pulp spread over its inner surface becomes organised, 

 and is added to this organ, so that it gradually acquires 

 greater thickness, the reason that the inner bark con- 



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