[VoL. 7 
8 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
as truly as may be the growth and health of the plants. Dried 
leaves and withered tips count for little. Average length of 
leafy shoot is included in some of the tables, but this affords 
merely an index of stockiness or attenuation of the plants. 
The purpose in this work was to get results not only in respect 
to growth relations with variations in the H-ion concentration, 
but also, especially in the later work, to secure data in respect 
to the extent of change in the Py of the medium in which the 
plants had grown. The time interval which the plants were 
permitted to remain in the cultures between changes of solution 
was generally 6 to 7 days, but in certain cases referred to later 
this interval was diminished or increased for special reasons 
indicated in connection with the tabulated data. During any 
interval, however, distilled water was added as required. In 
general, it seemed to the writer that the practical value of a 
particular mineral nutrient solution should rest in part upon its 
capacity to furnish favorable growth conditions for a period not 
too limited, that is, the solution should possess among other 
favorable characteristics, if possible, the quality of resisting 
unfavorable change over a period of one week or even longer. 
Had the nutrient solutions used been merely solutions A and 
C, in which phosphate is supplied as the acid salts, it would 
have been simple to change the H-ion concentration towards 
neutrality by the use of various proportions of a dibasic salt. 
However, it was at the same time desired to increase the H-ion 
concentration, that is, to diminish the Py exponent in order to 
determine approximately the favorable Pg limits. Moreover, 
changes in solution B could not be readily effected in the manner 
indieated without material changes in composition. It was 
therefore determined to use, in general, 0.1 n NaOH and approx- 
imately 0.1 molar H;PO, in shifting the H-ion concentration. 
Actually, the concentrations employed were 0.1 n NaOH and 
0.092 molar H;PO,. The stock solutions were so arranged as to 
concentration that the amounts of alkali or of acid introduced 
might first be diluted considerably with distilled water. In this 
way there was less danger of precipitating out the caleium salt, 
as insoluble calcium phosphate, for example, in solution A, 
before the critical Pg was reached, also other possibilities of 
precipitation in the case of solution B. 
