THE SECRETION OF INVERTASE BY PLANT ROOTS 
377 
An incubation experiment was also made. 500 cc. samples were taken 
from the culture solution and from the control solution, and toluene was 
added. The solutions were incubated at 35° C. and then again analyzed for 
reducing sugars. The increase after nine days was but 5 milligrams. 
Nutrient solution minus iron salts. Rice and Osugi (8) in working on 
the inverting power of various soils have presented evidence that inversion 
of sucrose may be affected by various colloids and suggest that the inversion 
may be due to adsorbed acids. In the Pfeffer's solution after sterilization 
there is precipitated ferric hydrate, and this precipitate is increased after a 
few days' growth of the plant. In order to determine whether or not the 
ferric hydrate might be responsible for the increase in reducing sugar, an 
experiment was performed in which iron was omitted from the culture solu- 
tion. The methods were the same as for the previous experiments. Sucrose 
was supplied at a concentration of 0.5 percent. Corn was again used and 
the plants were grown for 18 days in the greenhouse. The dry weights of 
tops and roots were 0.725 grams and 0.200 grams respectively. Analyses 
showed 500 milligrams of reducing sugars, while the control solution had 
only 305 milligrams. The increase was therefore 95 milligrams. 
Discussion 
What is the cause of the increase in reducing sugar in the culture medium? 
Is the enzyme invertase excreted? The evidence is contrary to this idea. 
In no case was there obtained any increase in reducing sugar after incuba- 
tion. It is possible, of course, that the enzyme invertase is excreted from 
the root in such small amounts that the reaction effected is very slight. 
It might be suggested, furthermore, that the culture solution is unfavorable 
to the invertase and that the latter is soon destroyed. It was noted, 
however, that whenever the culture solution became contaminated with a 
yeast or a fungus, there was a marked increase in reducing sugars, and that 
this increase continued after incubation. The incubation experiment for 
culture number 8 of experiment 2 yielded data in support of this statement. 
In accordance with the view of Rice and Osugi (8) it might be expected that 
the mucilaginous matter of the root and surrounding the root-cap cells as 
well as the cell walls might adsorb basic ions, the process resulting in a 
preponderance of hydrogen ions which might cause inversion of sucrose. 
But since the culture solution becomes increasingly alkaline in reaction with 
the advent of time, and since this alkalinity is due to the absorption of 
anions by the roots, it is reasonable to conclude that the zone about the 
roots is constantly of greater alkalinity than the "outer" regions of the 
culture solutions. In other words, the gradient of concentration of hydroxyl 
ions falls with increasing distance from the roots. 
There is still another alternative. The cells of the root-cap are sloughed 
off, and it might be suggested that the root cells in dying yield reducing 
sugar to the culture solution. But, as stated in another paper (Knudson, 5), 
