146 PHYSIOLOGICAL VIEWS. 



system. We will consider the sap as bearing a resemblance 

 to the animal chyle, and the leaves to the animal lungs. These 

 vegetable lungs are furnished with pores, by which they, too, in- 

 hale gases; but here our comparison fails, since, instead of ox- 

 ygen, the plant inhales carbonic acid ; this it decomposes ; and 

 converting to its own use the carbon, which is an important el- 

 ement of vegetable compounds, it exhales the oxygen necessary 

 for the support of animal life. Light, however, is necessary for 

 this process of respiration in the plant ; deprived of this agent, 

 vegetables absorb instead of giving off oxygen. 



The carbon, which is deposited in the sap, seems, in order 

 to be fitted for the nourishment of the plant, to require the far- 

 ther agency of oxygen, to convert it into carbonic acid ; this 

 is done by means of the oxygen, which, during the light, is ab- 

 sorbed by the leaves. At the appearance of light, carbonic 

 acid is again decomposed and oxygen evolved. Besides the 

 oxygen which the plant separates from the carbonic acid in- 

 haled by its leaves, it is undoubtedly furnished with this gas by 

 the decomposition of water* and other substances which are ab- 

 sorbed by the root. 



The cambium is the sap elaborated by the chemical process 

 carried on in the leaves, and rendered fit for the nourishment 

 of the plant. 



In tracing the descent of the cambium or returning sap, wo 

 shall not find it passing through the same vessels by which it 

 ascended ; it is chiefly conveyed by a system of vessels be- 

 tween the liber or inner layer of bark, and the alburnum or 

 young wood ; here it contributes both to the formation of new 

 wood and new bark, and extending from the extremity of the 

 roots, to the upper extremity of the plant, it furnishes materials 

 for the formation of new buds and radicles. 



If a ring is cut through the bark of a tree, the cambium will 

 be arrested in its course, and accumulating around the upper 

 edge of the bark, will cause a ridge or an annular^ protube- 

 rance. This vegetable blood being thus prevented from hav. 

 ing access to the lower part of the plant, the roots cease to 

 grow, the sap ascends but feebly, and the tree dies in two or 

 three years. If the incision is not made too deep, the wound 

 will soon heal by the union of the disconnected bark, and the " 

 circulation of the cambium proceeds as before. This experi- 



* Water consists of oxygen'm union with hydrogen, 

 t From the Latin word annulus, a ring. 



In what respect does the comparison between the respiration of plants and 

 animals fail ? What is needed in order to fit the carbon for the nourishment of 

 the plant ? Cambium, or descending sap How conveyed Importance of this 

 fluid What is the effect of cutting a ring through the bark of a tree ? 



