RETURN OP THE SAP. 35 



ate extensive cavities in the interior of the body, or pene- 

 trate through the interstices of a cellular tissue. That this 

 latter is the mode of transmission adopted in the vegetable 

 system has been considered probable, from the circumstance 

 that the nutritious juices are diffused throughout those plants 

 which contain no vessels whatsoever with the same facility 

 as throughout those which possess vessels; from which it 

 has been concluded that vessels are not absolutely necessary 

 for the performance of this function. The nature of the 

 forces which actuate the sap in its descent from the leaves, 

 and its distribution to different parts, is involved in equal 

 obscurity with the nature of the powers which contribute 

 to its motion upwards along the stem, from the roots to the 

 leaves. In endogenous plants the passage of the sap in its 

 descent, is, in like manner, through those parts which have 

 been latest formed; that is, through the innermost layers of 

 their structure. 



The returning sap, while traversing these several parts of 

 the plant, deposites in each the particular 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 

 contributes: 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 consequence, produced, 

 which, from the intricacies 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 results, namely, the 

 gradual deposition of the materials of the future alburnum 

 and liber. These materials are first deposited in the form 

 of a layer of glutinous substance, termed the Cambium; a 

 substance which appears to consist of the solid portion of the 



