PROGRESSION OF THE SAP. 149 



Draper attributes the movement of the sap to capillary attraction, 

 which he considers as an electrical phenomenon. This attraction takes 

 place when a fluid moistens a capillary tube, and there can be no flow 

 imless a portion of this fluid is removed from the upper extremity ; 

 for capillarity wUl 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 diflFerent 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 capUlary tubes), that will pass through most rapidly which wets 

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

 explains the phenomena of endosmose and exosmose by referring them 

 to capillary attraction, aided by transpiration. 



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 fad« from want of a due supply to 

 compensate for transpiration ; and hence the importance of pruning plants 

 before transplantiag 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 process of transpiration, therefore, by forming a vacuum, assists 

 capillary attraction and the atmospheric pressure, and thus the fluids 

 rise. As the process of endosmose and exosmose depends on the 

 chemical affinity and physical character of the fluids on each side of a 

 membrane, the porosity of the membrane, and the attraction existing 

 between it and either of the fluids, it follows that the nature of the 

 parietes of the cells and vessels of plants must have a marked effect 

 on their contents and secr'etiona. 



The observations of physiologists and chemists thus lead to the 

 conclusion that there are four factors concerned in the circulation 

 of the sap m plants — viz. nutrition, acting as a. vis a fronte, as is 

 shown by the current setting most strongly in the direction of most 

 rapid growth ; osmose, indicated by the difference in density between 

 the fluids of the plants and those supplied to it from without ; 

 capillary attraction, consequent on the character of the vessels ; and 

 lastly, evaporation, by which the capillary attraction is kept up, 

 osmose favoured, and nutrition facilitated. To these another may be 

 added, — intermittent mechanical strain, produced by swaying in the 



