12 Bouyoucos, Transpiration of Wheat Seedlings as Affected etc. 



The increase in the relative transpiration with tlio decrease 

 in density down to the point whore the change takes place, may 

 be attributed to three factors; namely, 1) to the different modi- 

 fication of the structurc oftheplants, broug'ht abont by the different 

 densities; 2) to the different osniotic 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 bej:>"inning of 

 the investigation that if too high densities were used, they would 

 not only prove harmt'ul to the plants, but they would also uiodify 

 their structure or anatoniy, 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 streng:th 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 

 diiferent Solutions, ünder 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 

 influence 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 growm. 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 

 proportionaUy, and consequently, the evaporation of the water from 

 the surface of the plants increases, and therefore, results in what 

 has akeady 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 eifect. This is easy to see. As the density of the Solution 

 decreases, its osmotic pressure diminishes almost proportionaUy, 

 and therefore, everything eise being equal, the ease or facility 

 with which the plants can absorb water increases in the same 



