ato) A TEXTBOOK OF GENERAL BOTANY 
pressure in them is increased. This increase in osmotic pressure 
results in the diffusion of water from the xylem of the veins 
to the chlorenchyma, to. replace the water lost by transpiration. 
The xylem is continuous from the leaf, through the stem, to 
those parts of the roots where water is absorbed. In the xylem 
water moves, not by diffusion, but as a steady stream through 
long tubes composed of dead cells. This stream of water carries 
the dissolved mineral matter along with it to the veins of the 
leaf, where both the water -and the mineral matter can diffuse 
into the cells of the chlorenchyma. The movement of salts from 
the veins into the chlorenchyma, being by diffusion, is independ- 
ent of the passage of the water but dependent on the relative 
- concentration of the particular kind of matter in the chloren- 
chyma cells and in the veins. The reason why the kinds of 
mineral matter that are used in the leaves continue to diffuse 
into the chlorenchyma is that in their assimilation they are 
combined with other compounds and are thus changed to dif- 
ferent substances. When a given substance combines with an- 
other to form a new compound, the concentration of the original 
substance is decreased. Owing to this fact a substance which 
is in solution in the xylem of the veins will continue to diffuse 
into the chlorenchyma as long as it is used by being combined 
with some other compound. The importance of transpiration in 
the passage of mineral matter into the leaves is that transpi- 
ration causes a stream of water to pass through the xylem and 
that this water carries dissolved material into the veins, where 
it can diffuse into the chlorenchyma. 
The rate of the movement of mineral matter into the leaves 
is not necessarily proportional to the rate of transpiration, as 
the movement of dissolved material from the xylem into the 
chlorenchyma is by diffusion and depends on the relative con- 
centration of the particular dissolved material in the two tissues, 
while the entrance of dissolved substances into the root is also 
by diffusion and depends on the relative concentration in the 
soil and root. Actual experiments indicate that under ordinary 
conditions there is little relation between rates of transpiration 
