206 CONVERSION OP ASCENDING SAP. 



earbon and hydrogen, there must be a superfluity of the first of 

 these ingredients. This oxygen is probably conveyed away by 

 the spiral vessels, which form the medullary sheath of Exogens, 

 and which are diffused through the woody bundles of the stem 

 of Endogens. These spiral vessels communicate with the leaves; 

 and through them, the oxygen is given off to the atmosphere. 

 By dividing stems under the surface of mercury,* and collecting 

 the minute bubbles of gas which arise from the cut ends of the 

 spiral vessels, it has been shown, that they contain a considerably 

 greater proportion of oxygen than exists in the atmosphere. 



325. The processes by which the crude sap conveyed to the 

 leaves, is converted into the " proper juice " of the plant, have 

 already been described in so much detail, that it is not necessary 

 to do more than briefly recapitulate them here. By Exhalation 

 and Evaporation, a great quantity of superfluous water is got rid 

 of ; and the fluid is thus concentrated. By the absorption of 

 carbon under the influence of light, the process to which the 

 name of Digestion may be given, a large quantity of solid 

 matter is added to it ; and the materials are afforded for the 

 increase of the woody structure, which requires this ingredient 

 in a peculiar degree. And by the process of Respiration is 

 removed the product of the slow decay of the whole structure, 

 which would be highly injurious if retained within it. 



326. We must remember, however, that, in speaking of these 

 functions, we only state the evident results of those more obscure 

 changes, which take place in the interior of the plant, and of 

 which the nature is still unknown. In all plants, the functions 

 of Exhalation, Digestion, and Respiration, are performed almost 

 in a similar manner ; and the materials upon which they operate 

 are (as already explained) nearly of a similar character ; and yet 

 the products are remarkably different. In nearly all of them, 

 however, the material of the tissues themselves is the same. 

 Woody fibre, for example, is found by the chemist to be com- 



* This fluid is used instead of water, when it is desired to collect gases which 

 might he absorbed by water. (See Treatise on CHEMISTRY.) In the present case 

 it is employed to prevent any carbonic acid that might exist in the vessels, from 

 being undiscovered, through the absorption of it by the water. 



