106 BIOLOGY. [Book ii. 



The Sap can descend by the aubier or by the ligneous tissue of the 

 central system. Knight having decorticated circularly a stem of 

 Solanum tuberosum, saw the subterranean tubes developing them- 

 selves indeed less, yet still developing themselves; in which 

 case the sap could only have descended by the bark. 



The same observer has demonstrated that the elaborated sap 

 can take an ascending movement when it is dissolved and carried 

 on by the lymphatic sap.^ 



Furthermore the lymphatic sap can take a descendent 

 movement, for example, when it forms in the leaves, absorbing 

 the water which moistens them. When we cut the stem of a 

 plant containing abundance of liquids, for example a milky plant, 

 we see the latex flowing froni the two surfaces of section There 

 is here a simple sap movement, determined by the elastic pressiort 

 of the canals primitively distended, turgid through the action 

 of the endosmosis. In sum, the sap, elaborated or not, passes 

 where it can. A root which had become naked Dutrochet cut, 

 during winter, lower down than a shoot which it had produced ; 

 he saw, the following spring, this shoot continue to live. The 

 development of the leaves had not yet taken place ; consequently 

 the shoot lived without elaborated sap, by the mere help of the 

 lymphatic sap, driven now from above to below. Knight having 

 cut ofE from a forward variety of Solanum tuberosum the runners 

 which produce the tubers, there resulted in the stem a plethora 

 of elaborated sap. The plant, which usually is not florescent, had 

 flowers, fruits, and even small tubers were developed on many of 

 the aerian parts of the plant. 



If in the spring we cut a vine-root we see the lymphatic sap 

 flowing from the superior or central tronqon as from an aerial 

 stem. 



Dutrochet observed that the trunk of a tree cut down during 

 winter and completely stripped of its branches presents never- 

 theless in spring an outflow of elaborated sap under its bark. 

 This sap existed therefore in the central part of the vegetal 

 1 Knight, Philosophical Transactions, 1805. 



