Bouyoucos, Transpiration of Wheat Seedlings as Affected etc. 
11 
cnltures. It will be seen that the transpiration per gram of dry 
matter increases in every one of the three cnltures with the de- 
crease in density, until a certain dilution is reached and tben de- 
creases with a further decrease in concentration. To state the 
same thing in the converse form, the relative transpiration rises 
with the concentration up to a certain point beyond which it de- 
clines with a further increase in density. The concentration at 
which the change occurs is different for the various cultures, but 
it lies between 93.5 to 375 parts per million. For the solution 
culture, it is at 187.5 P. p. m.; for the sand culture, it is 375 
P. p. m.; and for the soil culture, it is at 93.5 P. p. m. The 
regularity with which the relative transpiration increases with the 
diminution in density and then declines with the further reduction 
' in concentration, is very remarkable for the sand and solution 
cultures, but in the soil culture, there is one exception. It will 
be observed that in both the water and sand cultures, there is not 
the slightest break in the curve from the highest density to that 
where the change takes place, nor from there to the lowest con¬ 
centration. In the soil culture, however, there is one irregularity 
in the curve before the density where the change commences, 
is reached. Undoubtedly this discrepancy would probably not be 
noticed had there been as many trials of this culture as there were 
of the other two. 
As far, therefore, as these results may be considered con- 
clusive, they go to show two facts; first, that the relative tran¬ 
spiration increases with the decrease in density, down to a certain 
point; and secondly, after this point, it declines with the further 
diminution in concentration. Evidently, the highest and lowest den- 
sities diminish it the most, and the intermediate ones the least. 
These results are significant not only on account of the relation 
they indicate between the density and the transpiration, but also 
because they throw some doubt upon the Statement made by certain 
investigators x ) that the total transpiration is as good a criterion 
for measuring plant growth as the weight of the plants. Indeed, 
the foregoing data tend to disprove this view most decidedly. They 
show that the media or density of solution in which the plants are 
grown, have a most significant effect upon the relative transpiration, 
and that the latter varies in some cases, between the different 
densities within the same culture, from 360.0 to 471.4 grams, or 
a difference of 11.4 grams, in view of these facts, therefore, it 
can hardly be considered, from these results at least, that the 
total transpiration is an accurate and true criterion of measuring 
plant growth, even though the total transpiration may be to a 
certain extent a function of plant growth. 
The next question to decide is, what forces cause these pheno- 
mena. It must be stated in advance that no definite and proved 
explanation can be offered; the following suggestions, however, 
may be worth considering. 
q Bot. Gaz. 40. 1905. p. 178—195. 
