264 LEAVES 



As the student well knows, the movement of water and dis- 

 solved substances into and out of living cells is in accordance with 

 the laws that govern the passage of liquids through membranes. 

 But in passing from roots to leaves and other parts of the shoot, 

 the water with the substances in solution passes through the 

 tube-like xylem vessels, which are composed of the cell walls of 

 dead cells, and in such cells, with cell membrane and all parts 

 of the protoplasm absent, the structural features upon which 

 osmosis depends are not present. Of course throughout the stem 

 and roots the osmotic activity of living cells around the xylem 

 may have something to do with the movement of liquids through 

 the vessels, but this force combined with capillarity and root 

 pressure seems entirely inadequate to carry water from the roots 

 to the tops of tall trees. That transpiration has much to do with 

 the movement of water through the xylem vessels has been quite 

 well demonstrated by a number of experiments. 



A column of water, due to the coherence of the water mole- 

 cules, holds together much like a thread or rope. The coherence 

 of water molecules is shown by the way water drops maintain 

 themselves when hanging on the end of a pipette or on the eave 

 of a building where, by accumulating and freezing while still 

 clinging, they form icicles. It has been demonstrated that even 

 very small columns of water, like those reaching from roots to 

 the leaves through the xylem vessels, are able to endure heavy 

 strains without breaking. Regarding the columns of water 

 through the vessels as small but tough threads with one end in 

 contact with the soil water at the roots and the other end in 

 contact with the cell sap in the mesophyll cells of the leaf, it is 

 evident that whenever water becomes scarce in the mesophyll 

 cells through transpiration, then by osmosis these columns of 

 water will be pulled in until the cells of the mesophyll are so filled 

 with water and their cell sap so diluted that they no longer have 

 the osmotic force to overcome the resistance of the water columns. 

 But since transpiration is practically continuous, although varying 

 much in rate at different times, the water columns are drawn into 

 the cells of the mesophyll almost continuously, and hence the 

 apparently continuous flow of water and dissolved substances 

 i hrough the xylem of plants. Thus, transpiration, by removing the 

 water from the cells of the leaf and thereby causing the dissolved 

 substances in the sap of these cells to become more concentrated, 



