314 AMERICAN JOUIINAL OF BOTANY [Vol. 9, 



Whether the suppression of foliar evaporation signifies the suppression of a water current in 

 the axis does not appear. 



The authors of several important textbooks of plant physiology — 

 Sachs (1887), Pfeffer (1900), and Jost (1907) — state that a "transpiration 

 current" is necessary for supplying adequate quantities of the neceseary 

 salts to the plant. These authors make statements to the effect that it is 

 more or less obvious that it is necessary for a plant to take up larger amounts 

 of a dilute solution than of a concentrated solution in order to supply the 

 salts necessary for growth. None of these authors presents any new data 

 nor even cites conclusive data which supports, his conclusions. It is true, 

 as McLean (1919) has pointed out, that the almost complete suppression 

 of transpiration does not necessarily signify the suppression of a water 

 current; but in this connection it should also be borne in mind that the 

 existence of a transpiration stream in the sense of mass movement of solu- 

 tion in normal growing plants has yet to be demonstrated. 



This brief review of a number of works which include some data or dis- 

 cussions of the relation of transpiration to the absorption of salts serves to 

 indicate that the data presented and the conclusions drawn are often con- 

 tradictory. It would be surprising indeed to find a group of workers ob- 

 taining the same results and arriving at the same conclusions under such a 

 variety of methods. It might be expected that ii; a study of the relation 

 between transpiration and the absorption of salts the results might differ 

 depending upon whether transpiration was varied by changing the atmos- 

 pheric humidity, light intensity, soil water, or concentration of the nutrient 

 solution. All these factors affect transpiration, but when one varies one 

 of these factors in endeavoring to reduce transpiration, he may also vary 

 some other factor or factors which may become the determining factor. 

 Perhaps a lack of consideration of the principle of limiting factors in addi- 

 tion to a lack of uniform methods and materials is largely responsible for 

 some of the contradictory results presented by various investigators. 



A few early investigators presented data which seemed to them to indi- 

 cate that there is a relation between transpiration and the absorption of 

 salts; others got no definite results. Teachers and investigators were in- 

 clined to accept the former results and to disregard the latter. Even the 

 writers of many of the physiological textbooks in use today make very defi- 

 nite statements regarding the relation of transpiration to the absorption 

 of salts with nothing except the fragmentary data of some of these earlier 

 workers and speculation upon which to base their statements. Results ob- 

 tained by some of the more recent workers indicate that there is not neces- 

 sarily a relation between the quantity of water transpired and the quantity 

 of salts absorbed. Perhaps other factors besides transpiration are more 

 important in determining how large are the quantities of salts absorbed by 

 plants. 



