116 ORIGIN OF THE WOOD. 



gestion abundantly compensales not only for the carbonic acid, 

 which they yield to the atmosphere!, but also for the vast amount 

 constantly given out by animals, and that produced by com- 

 bustion. They not only yield oxygen, but ihey take the nox- 

 ious gas and decouipose it, and retain the carbon and return 

 to animals the vilal air. Thus the two great kingdoms of 

 animated nature mutually yield to each other its vital fluid. 

 Were it not for this compensative arrangement we see no other 

 result, constituted as the beings of this glohe now are, that 

 could hapj)en, but that tlie atmosphere would become so vitiated 

 as to be until for the support of animal existence. How beau- 

 tifully they now operate. The expelled carbonic acid, whicii 

 is poisonous to the animals, which throw it off in vast quanti- 

 ties, is taken up by the vegetation as the most healthful and 

 appropriate food for thtmi, and the oxygen which they do not 

 require by their constitution gives life to man. 



That very important operations take place in the leaves is 

 conclusively proved by many observations on the leaves of 

 different plants at diflerent periods of the day. Hayne found 

 the Bryophyllum calcynum to be acid in the morning, taste- 

 less at noon, and bitter in the evening. The same is true of 

 many other leaves, and some will even redden litmus paper in 

 the morning, but produce no effect upon it at any other part 

 of the day. 



Section 3. Origin of the Wood. 



145. We might occupy pages in the various discussions which 

 have been carried on concerning the origin of the wood. 

 From the time of Linnaius to the present, various theories 

 have been formed and advocated with spirit ; but the discovery 

 of new facts has compelled, in some cases, tlie authors to yield 

 their favorite opinions and fall in vvith views more in accord- 

 ance with what at least appears to be nature's operation. We 

 shall not occupy our space on exploded theories, but proceed 

 directly to lay before the student the one which seems to bid 

 fair to supplant ail others. 



This tlieory supposes two distinct simultaneous systems of 

 growth : the cellular and tibro-vascular, of which the Ibrmer is 

 horizontal, and the latter vertical. The cellular gives rise to 

 the pith, medullary rays, and the remaining cellular substance 

 of the wood and bark. The fibro-vascular system gives rise 

 to the vascular portion of the plant. All the woody portion 

 of the trunk are the roots of leaves imbedded in the cellular 

 system by which they are confined. The buds differ from the 



