ITS ASCENT AND DESCENT. 133 



These are questions which have still to be answered. 



But upon yet another question we may dwell at some 

 detail. What is the cause of the circulation of the sap ? 



To the best of our knowledge, this important problem 

 has never been propounded as it should have been. And 

 for this reason : all observers who have taken up its con- 

 sideration have had in view only the rising sap, and the 

 cause of its rising. Evidently this is but a $ari of the 

 problem. The ascending sap, after undergoing an important 

 modification in the leaves, becomes the descending sap ; just 

 as the venous blood is transformed, on coming into contact 

 with the air in the lungs, into arterial blood. It is this 

 alternative movement of going and coming which constitutes 

 the circulation both of the sap and the blood, and which 

 ceases completely only with the life of the plant or the 

 animal. We must, therefore, bear in mind, which has not 

 been hitherto done, these two opposite, yet indissolubly 

 connected, movements, before we can approach with advan- 

 tage the solution of the proposed question.* 



Science consists in discovering, among the different ways 

 of looking at things which present themselves to the mind, 

 the one which appears to explain most clearly the phe- 

 nomena submitted to observation. He who doubts the 

 accuracy of our remark need only join us in reviewing 

 the different opinions enunciated up to the present time 

 on the cause of the rise of the sap. 



* The best means of ascertaining the coexistence of an ascending and de- 

 scending sap have been indicated in " The Circle of the Year," pp. i6v8. 



