174 J- J- SKINNER AND F. R. REID 
viz., the mono-sodium phosphate (NaH2P04) which Hke calcium acid phos- 
phate is acid in reaction, the di-sodium phosphate (Na2HP04) which is 
neutral; and the tri-sodium phosphate (Na3P04) which is alkaline in reac- 
tion. In all other respects the culture solutions were the same in concen- 
tration and in composition as in the first experiment described where calcium 
acid phosphate was used as the source of phosphate. In each of the three 
following experiments, one of the sodium phosphate salts was substituted for 
the calcium phosphate. 
Effect of Mono-sodium Phosphate 
The green weight of the wheat plants in the two sets of nutrient cultures 
are given in the two charts of figure 6. In both of these triangle sets, 
mono-sodium phosphate (NaH2P04) was used as the phosphate salt, while 
the nitrate and potash salts were the same as in the former experiment. 
The crotonic acid was used in a concentration of 25 parts per million. The 
plants as before grew for 12 days; the solutions were changed three times. 
Fig. 6. Green weight in grams of wheat plants in nutrient solutions containing varying 
ratios of phosphate, nitrate, and potash, the source of P2O5 being mono-sodium phosphate, 
(i) Without crotonic acid, and (2) with 25 p. p.m. crotonic acid. 
The harmfulness of the crotonic acid is again demonstrated in this 
experiment. Its relative toxicity in solutions of dififerent composition is 
again shown to vary. It was again least harmful in the solutions containing 
the higher amounts of P2O5. This is shown by examining table 4. 
In general the mono-sodium phosphate had somewhat the same efTect 
as the mono-calcium phosphate ; the higher the amount of phosphate in the 
culture solution the slighter the toxicity of the organic acid. 
When the cultures are grouped, so as to bring together all those in the 
phosphate end of the triangle, and all those in the nitrogen end, and all 
those in the potash end, so as to compare each group of cultures in the 
normal triangle with the similar group in the crotonic acid triangle, it also 
