106 OEIGIN OF THE WOOD. 



of their digestion abundantly compensates not only for the car- 

 bonic acid which they yield to the atmosphere, but also for the 

 vast amount constantly given out by animals, and that pro- 

 duced by combustion. They not only yield oxygen, but they 

 take the noxious gas and decompose it, and retain the carbon, 

 and return to animals the vital air. Thus the two great king- 

 doms 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 globe now are, 

 that could happen, but that the atmosphere would become so 

 vitiated as to be unfit for the support of animal existence. How 

 beautifully they now operate ! The expelled carbonic acid, , 

 which is poisonous to the animals, which throw it ofl" in vast 

 quantities, is taken up by the vegetation as the most healthful 

 and appropriate food for them, 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 different periods of the day. Hayne found 

 the Bryophyllum calcynum to be acid in the morning, tasteless 

 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. 



188» We might occupy pages in the various discussions 

 (\'hich have been carried on concerninsf the orio;in of the wood. 

 h'ovcv the time of Linnseus to the present, various theories have 

 )een formed and advocated with spirit; but the discovery of 

 lew facts has compelled, in some cases, the authors to yield 

 ,heir favorite opinions, and fall in with views more in accord- 

 ince with what, at least, appears to be nature's operation. 



189. There are now two systems which divide botanists as to 

 the origin of wood. One of these theories supposes two dis- 

 tinct simultaneous systems of growth — the cellular and fibro- 

 vascular, of which the former is horizontal, and the latter verti- 

 cal. 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 im- 

 bedded in the cellular system by which they are confined. The 



What relation have animal and vegetable respiration to each other ?— 

 188. What is said in relation to the origin of wood ? — 18y. How man} 

 systems ? What does one of tl ese suppose ? 



