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 
through 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, 
