16 Bouyoucos, Transpiration of Wheat Seedlings as Affected etc. 



once siiggest tlie nicthod aircady dcscril)C(l of determining- the 

 concentration of the soll Solution. W'hether it would be a practical 

 way, however, is another (lucstion. 



This, in short, is the manncr in which the osniotic pressure 

 of the sohition and the density of the cell sap may cause the re- 

 lative transpiration to increase with the decrease in density ot the 

 Solution down to a certain point. The decrease of the correlative 

 transi)iration froni this point down, i. e., with the further decrease 

 in density of Solution, may be attributed to another factor, naniely, 

 to the decreased power of certain densities of Solution to stimulate 

 transpiration. It is assunied here that the lowest density of the 

 nutrient Solution has a slight stimulative action upon transpiration, 

 and that it increases with the increase in concentration up to a 

 certain point, beyond which it ceases, or is overcome by the os- 

 motic pressure; or dccreases from this point with the diminution 

 in density, just as has already been noted. In other words, the 

 decreased transpiration per unit of growth from the intermediate 

 to the most dilute Solution, is ascribed to the reduced stimulative 

 action of the decreased density of Solution. To State the same 

 thing- in the converse form, the rise in the relative transpiration 

 from the most dilute to the intermediate densities, is due to the 

 greater stimulative action of the increased concentration. The 

 stimulative action cannot continue much further upward than where 

 the chang-e in the curve occurs, because it either reaches its 

 maximum, or it is O'Vercome by other factors, namely, the osmotic 

 pressure of the Solution, and the different physical properties of 

 the cell sap. 



In what manner the trans])iration is stimulated or accelerated 

 by the dilute Solutions is not known, it may be chemical, physio- 

 logical or both. 



The Avhole problem may now be stated in either of the 

 following ways: 1) the transpiration per unit of growth increases 

 with the increase in density of Solution, due to the stimulative 

 action of the latter, until a certain concentration is reached and 

 then begins to decrease with a further rise in density, due to the 

 increased osmotic pressure of the latter and to the increased density 

 of the cell sap; or 2) the relative transpiration increases with the 

 decrease in concentration down to a certain point, due to the re- 

 duced osmotic pressure of the dimished densities of Solution and 

 to the decreased power of the cell sap to rcduce evaporation, and 

 then it begins to decrease with a further decline in density, due 

 to the reduced stimulative action of the latter upon it. 



Going back to the original tables (I — III) of Solution, sand, and 

 soll cultures, it is very signiflcant to note that while the relative 

 transpiration increases down to a certain point and then decreases, 

 the total dry matter produced increases in all three cultures with 

 the rise in density. In other words, the actual plant growth is 

 proportional to the concentration of the Solution. This ratio is 

 certainly remarkable for its regularity, being perfectly constant 

 without any iuterrui)tion in the curve from the lowest to the hig- 

 hest density in all three cultures. 



