1914] BRIEFER ARTICLES 529 
method which has been almost exclusively used is to express the juice 
or to make watery extracts of the ground or minced tissue. The ability 
of these extracts to accelerate the oxidation of such reagents as gum 
guaiac, a-napthol, hydrochinone, etc., has been taken as an indication 
of the presence of oxidases in the respective tissues. 
The extracts which appear to be without oxidases when tested in this 
manner have, in the majority of cases, a distinct acid reaction. This 
fact was noted by CLark3 in summarizing the results of his own experi- 
ments. He concluded that extracts having an acidity such that 10 cc. 
required 8 cc. 0.1 N, KOH to neutralize to ppenciphsnaiye were with- 
out oxidases. 
The writer, proceeding in a similar manner with a considerable 
number of acid tissues, including several not previously tested, obtained 
results comparable with those reported by CLarkK. He then made a 
series of special tests on Citrus fruits, probably the most acid of tissues, 
using a different method of examination, which seems to throw light on 
the general condition in acid tissues. Lemons, oranges, grape fruit, and 
kumquats were tried. In these fruits the juice is contained in long and 
enlarged sacs attached to the inside of the carpel walls. From this 
point of attachment they extend into the carpel, giving it the familiar 
honeycomb-like appearance. These sacs may be readily separated 
from the carpels and from one another without injuring their skin, which 
consists of 2-10 layers of cells. In this condition, when placed in a 
solution of gum guaiac, a-napthol, sodium selenite, or similar reagents, 
along with hydrogen peroxide, the surface of the hairs soon becomes 
covered with the colored compound resulting from the oxidation of the 
reagent. This indicates the presence of oxidases (or, according to the 
terminology of Bacn and Cuopat, peroxidases, since hydrogen peroxide 
was required) in the cells of the sacs which contain the juice. 
It is thus evident that Citrus fruits have normal oxidases in their 
acid tissues. It is also evident that these oxidases are protected in some 
manner from the action of the acid which at this concentration effec- 
tually inhibits the action of oxidases. It seems to the writer that this 
protection may be afforded by a semipermeable surface (the plasma 
membrane or cell walls similar to the cell walls of barley seed, which are 
impermeable to acid) through which the acid is unable to pass. When 
the tissue is ground, previous to pressing out the juice, the structure 
which separates the acid from the ferment is destroyed, so that the action 
of the latter is inhibited. That these membranes are not normally 
3 CLARK, Torreya 11:1oI. 1gtl. 
