12 
Bouyoucos, Transpiration of Wheat Seedlings as Affected etc. 
The increase in the relative transpiration with the decrease 
in density down to the point where the change takes place, may 
be attributed to three factors; namely, 1) to the different modi- 
fication of the structure of the plants, brought about by the different 
densities; 2) to the different osmotic pressure of the various con- 
centrations; and 3) to the different densities of the cell sap of the 
plants grown in the various densities of solution. 
As to the first factor, it was realized at the beginning of 
the investigation that if too high densities were used, they would 
not only prove harmful to the plants, but they would also modify 
their structure or anatomy, i. e., cause them to assurae the xero- 
phytic form, which would tend to reduce transpiration, and thus 
destroy the central object of the reserach. The concentrations 
employed, therefore, were of a strength below that which would 
bring about the above conditions. Furthermore, from a large 
number of examinations, there were never observed any noticeable 
differences in the external structure of the plants grown in the 
different Solutions. Under these conditions, therefore, it seems that 
the increase of the relative transpiration with the diminution in 
density, is not due to any difference in the outer anatomy of the 
plants grown in the various concentrations. 
In the second factor it is conceived that in the highest con- 
centrated solution there is a high osmotic pressure but not so high 
as to cause plasmolysis. This high osmotic pressure has a limiting 
infiuence in the amount of water the plants can absorb. As this 
high osmotic pressure decreases, however, by the decrease in the 
density of the solution, the amount of water the plants can take 
up, increases. The ultimate result of this would be what the 
results already indicate. 
In the third factor, it is assumed that the density of the cell 
sap of the plants increases with the concentration of the solution 
in which they are grown. That is to say, the density of the cell 
sap of the plants grown in the highest concentrated solution is 
greater than that of the plants grown in the less dense solution, 
and that it decreases in direct ratio with the diminution in density 
of the solution. In this condition, the high concentration of the 
cell sap in the high density of solution, would possess very high 
osmotic pressure, viscosity, and low vapor tension, all of which tend 
to produce the same effect namely, to retain water. As the density 
of the cell sap decreases, however, the degree of magnitude of 
transpiration which these physical properties can diminish decreases 
proportionally, and consequently, the evaporation of the water from 
the surface of the plants increases, and therefore, results in what 
has already been mentioned. 
It must have already been seen that both factors may and 
can act together at the same time, for both tend 'to produce the 
same effect. This is easy to see. As the density of the solution 
decreases, its osmotic pressure diminishes almost proportionally, 
and therefore, everything eise being equal, the ease or facility 
with which the plants can absorb water increases in the same 
