1915] LONG—GROWTH IN CACTI 495 
due to imbibed water. It may be assumed that the osmotic pres- 
sure of the less ionized malic acid in N/50 concentration would 
also be without appreciable effect in the experiments in question. 
TABLE II 
Solution | Wt. at start | Wt. after 24 hrs. { Percentage increase 
Ristiled Water. ot 17.64 gm. 22.59 gm. 28.1 
PO EG INO ok baa ies We 17.83 2.88 28.3 
N/to0 KNO, ies meee ee 15.65 oak 23.9 
N BNOi con 16.75 13079 —17.8 
It seems probable, in view of these facts, that the lowered swel- 
ling in acid solution is the direct effect of the acid ion upon the 
colloids which take part in the swelling. The slower growth rate 
in acid solutions is significant in this connection. 
The action of alkalies upon swelling was not so regular in its 
effect as that of acids. While a swelling of lower degree than that 
produced in distilled water was practically invariable with acids, 
the swelling produced in alkalies was sometimes greater and in 
other cases less than that taking place in distilled water, and no 
satisfying explanation can be given for this behavior. The vary- 
ing acidity of the plant itself was perhaps a factor, by neutralizing 
to a greater or less degree the alkali of the penetrating medium. 
The curve represents the average of all cases examined. 
It is interesting to observe that in nutrient solution, practically 
always a slightly increased swelling over that taking place in dis- 
tilled water occurred, and furthermore that this was paralleled, 
as might be expected, by a greater growth rate in this medium. 
The mechanism of this action, which took place in a neutral medium, 
was not determined. 
In general, growth and swelling in these experiments paralleled 
rather closely nutrient solution exerting an accelerating effect 
over that observed in distilled water, and hydrochloric and malic 
acids an inhibitory one, while the effect of sodium hydroxide was 
irregular. 
While no attempt was made to separate and determine the 
~ colloid constituents of the plant to the presence of which the swelling 
