17, 6 Trelease: Salt Requirements of Wheat Plants 537 
thesis and other processes, may be greatly modified by the way 
in which the leaves are exposed to the air. The pointed end of 
a cylindrical wooden rod (a quarter-inch dowel), 6 millimeters 
in diameter and 30 centimeters long, was set into a hole in 
the center of the cork. A loop (5 centimeters in diameter) of 
paraffined copper wire, the lower part of which was wrapped 
around the rod several times, was used to hold the plant erect.** 
As the height of the plants increased, the loop was raised on 
the rod. Care was taken to support all the plants in the same 
way. ; 
The same aérial conditions were secured for all the cultures 
in a series by rotating the cultures near the margins of the cir- 
cular tables described by Shive. Each of the cultures was thus 
exposed to approximately the same changes in light, temper- 
ature, and humidity conditions. 
The frequency with which the culture solutions are renewed 
is very important in experiments with solution cultures.** The 
solutions here used were changed every four days. Thus the 
period between changing is not strictly comparable with that 
used by Shive and by Tottingham, each of whom employed a 
three-day period. When the solutions were changed the decrease 
in volume of the solution during the previous period was meas- 
ured, and the amount of solution found to have been removed 
was taken as an approximate measure of the amount of water 
absorbed and transpired during the period. 
The developmental stage of the plants probably has an im- 
portant bearing on their salt requirements.** The present study 
is concerned only with the early vegetative stage of the young 
wheat plants. When transferred from the germinators to the 
*For photograph see McCall, A. G., A new method for the study of 
plant nutrients in sand cultures, Journ. Amer. Soc. Agron. 7 (1915) 
250-252. 
* Trelease, S. F., and Free, E. E., The effect of renewal of the culture 
solutions on the growth of young wheat plants in water-cultures, Johns 
Hopkins Univ. Circ. N. S. No. 3 (March, 1917) 227 and 228. Merrill, M. C., 
Some relations of plants to distilled water and certain dilute toxic sub- 
stances, Ann. Missouri Bot. Gard. 2 (1915) 459-606. Stiles, W., On the 
interpretation of the results of water culture experiments, Ann. Bot. 30 
(1916) 427-486. Brenchley, W. E., The effect of the concentration of the 
nutrient solution on the growth of barley and wheat in water cultures, Ann. 
Bot. 30 (1916) 77-90. 
*“ Pember, F. R., Studies by means of both pot and solution cultures of 
the phosphorous and potassium requirements of the barley plant during 
its different periods of growth, Bull. Agric. Exp. Sta. Rhode Island State 
College 169 (1917) 1-50. 
