Oct., 1922] TURNER EFFECTS OF CERTAIN MINERAL SALTS 
441 
importance upon composition of the nutrient solution. It would seem that 
when great emphasis is placed upon concentration as the cause in altering 
the ratio of tops to roots, as has been done by certain English workers in 
this field, greater care should be taken in checking up solutions used in 
order to make sure that there are sufficient mineral nutrients present at all 
times in the solution, for we cannot avoid the conclusion that one or more 
ions may become limiting factors in these more dilute solutions. Hoagland 
has shown that NO?, PO4, and K may be exhausted if the solution is run 
long enough without renewal. Starvation and concentration effects may 
be entirely different, yet their outward manifestation may often be in- 
distinguishable. 
The hydrogen-ion concentration was carefully checked for each set of 
experiments. The pH value ranged from 5.59 to 6.47. Though the low- 
nitrate solution had the highest hydrogen-ion concentration when it had also 
the highest total concentration (3,075 p. p.m.), this hydrogen-ion concentra- 
tion could not be a factor in determining the ratio of tops to roots of plants 
growing in it, since the ratio is of the same relative order in respect to those 
of medium-nitrate and high-nitrate solutions as it was when the hydrogen- 
ion concentrations of the three solutions had the same value. 
Proof is brought forth in the pure-culture experiments that nitrates have 
no direct checking effect upon roots, but that, on the other hand, the growth 
of roots is increased directly with the nitrate content of the solution, pro- 
vided the disturbing influence of tops is not at hand. Thus, since there is 
no direct checking action of nitrates upon root growth, we shall have to 
explain the increased ratio of tops to roots on some other basis. Miss 
Brenchley (2) has offered an explanation of this phenomenon as follows : 
It seems as though the plant makes every endeavor to supply itself with adequate 
nutriment, as if, when the food supply is low, it strives to make as much root growth as 
possible so as to offer the greatest absorbing surface for whatever nutriment may 
be available. 
Realizing that this explanation was not adequate, she has later offered a 
more elaborate explanation (3). Both explanations are purely teleological 
and do not give the plant physiologist any satisfaction as to the mechanism 
by which the tops in plants come to make such increase over roots. 
Livingston's (17) explanation, which is based upon his experiments with 
plants growing in poor soil, is that the poor soil brings about such morpho- 
logical changes in the root as to render it incapable of absorbing water, and 
thus, that the growth of tops is limited because of an inadequacy of water. 
Curtis (36) has suggested that the effect of nutrients, especially of 
nitrates and phosphates, upon root growth might result, not from any 
direct action on or in the roots, but from an indirect effect caused by a 
change in the supply of food available for roots through the effect of the 
nutrient upon the use of the food by the tops. 
In the experiments studied here, we find the increase of ratio of tops to 
roots to be due to nitrate ions. It has been shown, further, that nitrates do 
