Bouyoucos, Transpiration of Wheat Seedlings as Affected etc. H 



cultures. It will be seen that the transpiration per gram of dry 

 matter increases in every one of the three cultures with the cle- 

 crease iu density, until a certain dilution is reached and then de- 

 creases with a further decrease in coneentration. To State the 

 same thing in the converse form, the relative transpiration rises 

 with the coneentration up to a certain point beyond which it de- 

 clines with a further increase in density. The coneentration at 

 which the change occurs is ditferent 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. na.; for the sand culture, it is 375 

 P. p. m.; and for the soll 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 coneentration, 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- 

 eentration. 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 coneentration. 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 1) 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 iu some cases, between the ditferent 

 densities within the same culture, from 360.0 to 471.4 grams, or 

 a dijffierence 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 deflnite and proved 

 explanation can be offered; the following suggestions, however, 

 may be worth considering. 



») Bot. Gaz. 40. 1905. p. 178—195. 



