34 THE VITAL FUNCTIONS. 



any visible vessels ; and they either permeate ex- 

 tensive cavities in the interior of the body, or pene- 

 trate through the interstices of a cellidar tissue. 

 That this latter is the mode of transmission adopted 

 in the vegetable system has been considered pro- 

 bable, from the circumstance that the nutritious 

 juices are diffused throughout those plants which 

 contain no vessels whatsoever with the same faci- 

 lity 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 dis- 

 tribution to different parts, is involved in equal 

 obscurity with the nature of the powers which con- 

 tribute 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 man- 

 ner, 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, deposits 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 with- 

 out 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 



