368 
ROBERT AARON STEINBERG 
also the partial precipitation of the heavy metals present in the soil solution, 
leads to increased growth both by decreasing their concentrations and by a 
marked diminution of the action of the heavy metals resulting from the 
increased alkalinity. 
My observations on the acceleration of growth by increased acidity, etc., 
are of interest in connection with MacDougal and Spoehr's (36) studies on 
the correlation between growth and the imbibition of water by colloids as 
influenced by variation in the acidity. Growth in Opuntia, they conclude, 
is decreased by the excessive accumulation of acids in darkness. The 
decreased acidity of the tissues due to the action of light during the forenoon 
is accompanied by an acceleration of growth. Furthermore, the imbibition 
by protein and carbohydrate colloids, and by mixtures of the two in varying 
proportions, differs as to acid, water, and alkali. The imbibition phenomena 
of gelatine, they find, parallel more closely the growth phenomena of the 
animal organism; whereas those of agar parallel more nearly the growth 
phenomena of plants. One of their colloidal mixtures compounded — gela- 
tine ICQ + agar i — reacts as regards imbibition in the presence of acid, of 
water, and of alkali in a manner greatly similar to the growth rates of A. 
niger under similar conditions of variation in the reaction of the medium. 
Thin plates of this mixture, they find, increase in thickness to 520 percent 
of the original thickness in n/ioo NaOH, to 750 percent in water, and to 
I ICQ percent in w/ioo HCl. 
It is always to be remembered, however, that in A. niger we have 
increased assimilation of food materials as shown by the increased dry 
weight. 
The phenomenon of imbibition of water by colloids has also been used 
by Fischer (15) in the interpretation of oedematous swellings in the animal 
organism. The imbibition of water by colloids like gelatine, gluten, etc., 
in the presence of various compounds and degrees of acidity is paralleled, 
he shows, to a high degree by the behavior of organized animal tissues 
under the same conditions. Whereas, however, the use by Fischer of 
certain acidities led presumably only to an increased water content, so that 
the total dry weight is not increased, in A. niger, as just noted, under analo- 
gous conditions an increase in the dry weight results. That perhaps increase 
in dry weight occurs to some extent through hydration of the colloids, as 
has been found by Pauli to take place with gelatine, is not, however, im- 
probable. Still, while there may be some question whether mere hydration 
can be referred to as growth, there can be no doubt that we have accelera- 
tion of true growth in ^. niger. 
The growth accelerations of A. niger resulting from "chemical stimula- 
tion" are hardly to be attributed in their entirety to, nor apparently are 
they paralleled by, the imbibitional capacity of the fungus for water. 
Growth as we are studying it involves cell multiplication and increase in 
weight of organic compounds. The root-tip has long served as the classical 
