40 THE VITAL FUNCTIONS. 



been latest formed ; that is, through the inner- 

 most layers of their structure. 



The returning sap, while traversing these se- 

 veral parts of the plant, deposits in each the par- 

 ticular materials which are requisite for their 

 growth, and for their maintenance in a healthy 

 condition. That portion which flows along the 

 liber, not meeting with any ascending stream of 

 fluid, descends without impediment to the roots, 

 to the extension of which, after it has nourished 

 the inner layer of bark, it particularly contri- 

 butes : that portion, on the other hand, which 

 descends along the alburnum, meets with the 

 stream of ascending sap, which, during the day 

 at least, is rising with considerable force. A 

 certain mixture of these fluids probably now 

 takes place, and new modifications are in con- 

 sequence produced, which, from the intricacy of 

 the chemical processes thus conducted in the 

 inner recesses of vegetable organization, we are 

 utterly baffled in our attempts to follow. All 

 that we are permitted to see are the general re- 

 sults, namely the gradual deposition of the mate- 

 rials of the future alburnum and liber. These 

 materials are first deposited in the form of a 

 layer of glutinous substance, termed the Cam- 

 bium; a substance which appears to consist of 

 the solid portion of the sap, precipitated from it 

 by the separation of the greater part of the water 

 that held it in solution. The cambium becomes 

 in process of time more and more consolidated, 



