30 APPLICATION OF THESE PRINCIPLES TO THE ASCENT OF THE SAP. 



the plant. During its descent the different vegetable principles necessary for the econ- 

 omy of the plant are removed from it, and a certain quantity goes down to the roots, 

 partly to aid in their growth, and partly to throw new qualities of ascending sap into 

 the tree. In this descent, the elaborated sap moves through a system of vessels which 

 anastomose with one another, in the same manner as the capillary vessels of animals. 

 These tubes go under the technical name of laticiferous tubes. 



91. We see, therefore, from this description, that there are two points of this circula- 

 tion which require attentive consideration the spongiole and the leaf. The spongi- 

 oles are nothing but the young succulent extremities of the roots, which have been re- 

 cently formed from portions of the descending sap, but that sap is itself a species of 

 mucilaginous solution. Precisely, therefore, as water will pass through the tissue of a 

 bladder, the interior of which is filled with gum-water, so will moisture from the ground 

 flow through the spongiole. There is no difficulty in accounting for the rise of the as- 

 cending sap on the principles of capillary attraction, and, indeed, this is the explana- 

 tion now generally received by vegetable physiologists. 



92. While, therefore, those philosophers have freely admitted the applicability of this 

 principle, under the name of endosmosis, to the explanation of the ascent of the sap, 

 they have attributed to it, as an aid, a force which conies into operation only in an 

 incidental way. This is the exhausting action of the leaf. But it is probable that this 

 force takes little or no part in the movement of the sap, for any tendency to a vacuum 

 occurring in the leaves of a tree would cause those structures to collapse, and not 

 eventuate in exerting suction power on the rising fluid. It is true, that if we take (67) 

 a branch covered with its foliage, and dip its cut extremity into water, imbibition of that 

 water will rapidly take place ; but the phenomenon is certainly not due, as is ordinarily 

 supposed by botanists, to the exhausting action of the leaf, but to a very different cause. 

 The exhaustion is only an incidental affair. 



93. Guided now by the principle we have laid down (88), let us predict what must 

 be the necessary action of the leaf. The ascending sap, which we will suppose, for 

 simplicity's sake, to be water, rises to the upper face of the leaf. It there obtains car- 

 bonic acid gas from the air, of which the sunlight effects the decomposition, the result- 

 ing action being a change from water to a mucilaginous solution. In the tissue of the 

 leaf we have, therefore, two fluids engaged, water and a mucilaginous solution ; and 

 what must of necessity be the result 1 The water will drive the mucilaginous solution 

 before it (84-88), and force it back, along its proper vessels, into the stem. The imbi- 

 bition, therefore, that we perceived when a branch is dipped in a vessel of water, docs 

 not arise, as the botanists say, from evaporation taking place on the leaf, but it conies 

 from the capillary reaction which is going on in the leaf between the water, which is 

 then presented as ascending sap, and the mucilaginous solution which has been formed 

 by the light of the sun. Evaporation, it is true, takes place, and comes into operation, 

 as I have said, in an incidental way, but the proper force which gives origin to the 

 whole phenomenon is the capillary action which is going on in the way just described. 



94. It is the imperfection of the principle on which they were relying- the exhaust- 

 ing action of the leaf that has caused botanists to look upon the descent of the sap 



