PROGRESSION OF THE SAP. 129 



their texture is often delicate, the movements are rapid. Many of 

 these, as sea-weeds, when plunged into water, after having been dried 

 by evaporation, imbibe the fluid with very great rapidity. 



266. The Cause of the Progression of the Sap has been investigated 

 by Malpighi, Hales, Dutrochet, Draper, Briicke, and Liebig. While 

 the capillarity of the vessels in the higher plants operates to a certain 

 degree, it would appear that the process of endosmose is that by which 

 the continued imbibition and movement of fluids is chiefly carried on. 

 From the loss of its watery contents, by exhalation, and the meta- 

 morphoses going on during the process of nutrition and secretion, the 

 sap becomes gradually more and more dense, and thus, throughout 

 the whole plant there is a forcible endosmotic transmission of the 

 thinner fluids, and a constant change in the contents of the cells and 

 vessels. These movements will of course take place with greater 

 vigour and rapidity according to the activity of the processes going on 

 in the leaves, which thus tend to keep up the circulation. 



267. Draper attributes the movement of the sap to capillary attrac- 

 tion, which he considers as an electrical phenomenon. This attraction 

 takes place when a fluid moistens a capillary tube, and there can be 

 no flow unless a portion of this fluid is removed from the upper ex- 

 tremity ; for capillarity will not of itself raise a fluid beyond the end 

 of the tube. Evaporation and transpiration, which take place in the 

 leaves, remove a portion of the vegetable fluids, and thus they promote 

 the capillary action of the vessels. When two fluids of different kinds 

 come into contact in a tube on different sides of a membrane, (which 

 membrane being porous, may be considered as made up of numerous 

 short capillary tubes), that will pass the fastest which wets it most 

 completely, or has the greatest affinity for it. Hence, Draper ex- 

 plains the phenomena of endosmose and exosmose by referring them to 

 capillary attraction, aided by transpiration. 



268. Liebig adopts a somewhat similar view of the phenomena. He 

 states that the accurate experiments of Hales have shown the effects 

 of evaporation and transpiration on the movements of sap. Transpira- 

 tion takes place chiefly in clear and dry weather, and consequently is 

 regulated by the hygrometric state of the atmosphere. When the 

 weather is cloudy and the atmosphere moist, transpiration is checked, 

 and stagnation of the juices takes place. The greater the transpira- 

 tion, the greater the supply of fluid necessary. Hence, plants kept in the 

 dry atmosphere of rooms fade from want of a due supply to compensate 

 for transpiration ; and hence the importance of pruning plants before 

 transplanting them, so as to diminish the evaporating surface, and of 

 performing the operation in dull and moist weather, so as to allow the 

 absorption of fluids to keep pace with the transpiration. This pro- 

 cess of transpiration, therefore, by forming a vacuum, assists capillary 

 attraction and the atmospheric pressure, and thus the fluids rise. As 



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