108 POPULAR EEEOES. 



more material continues through the growing 

 season. We now see, since the most of the soliS 

 material comes from above, why it is that the 

 greater growth takes place upon the upper side of 

 a wound in the trunk of a tree. The food material 

 from the leaves which has gradually diffused down- 

 ward from cell to cell, would cause the sap to be 

 richer upon that side of the cut than it would be 

 below. The causes which lead the sap to enter the 

 plant, and continue to do so while it is in a grow- 

 ing condition, are somewhat complicated, and per- 

 haps not yet fully understood. Two very important 

 influences at work, however, are osmose and evapor- 

 ation. If a glass tube, having its lower end cov- 

 ered by a membrane, be partly filled with a solution 

 of salt, sugar, or some other material, aod placed in 

 a vessel of water, the solution will gradually pass 

 outward into the water; but the water will also 

 enter the tube and dilute the solution, and this will 

 occur so rapidly that the solution will rise in the 

 tube above the level of the water outside. Upon 

 the same principle the moisture of the soil enters 

 the plant and tends to dilute the sap already 

 present, but as the evaporation which takes place 

 from the leaves tends continually to condense this 

 sap, a constant inflow of sap takes place from the 

 soil. A striking illustration of the fact that the 

 difference in density of the fluids in the plant and 



