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sap had been expressed; yet, upon the collapse of the wall, the delicate protoplasmic layer inevitably became loosened, and, as bits of protoplasm appeared in the remaining drop of sap, it was always included in the analyses. Since the concentration of arsenic was, in every case, considerably less in the sap than in the protoplasm, the amount obtained by including a drop of sap in the analyses of the protoplasm is within the limits of experimental error. Centrifugating the protoplasm free from the sap could not be relied upon on account of probable exosmosis of substances from the protoplasm into the sap, with consequent changes in the volume of the remaining substances. It is hoped that a more accurate method may be devised by further experimentation. Since the error will be the same in all cases, however, it can not, in any event, affect conclusions as to the relative
distribution of different arsenic compounds. In calculating the amount of arsenic in the wall and the protoplasm the experimental results were multiplied by the factors 257 for wall and 1U4 for protoplasm.
In connection with this problem on penetration of substances it was thought of interest to find out whether the wall becomes thicker with age, by comparing the ratio of the weights of wall and sap of various sized cells. In comparing the weights' of sap and wall it must be remembered that the volume of the sap increases as the cube, and the wall surface increases only as the square, of the diameter of the cell. If the wall remained of the same thickness, the ratio of cell wall to sap would differ with the size of the cell, as shown in Figure 3, curve A. Curve B shows the experimental results. There are slight systematic deviations from the theoretical curve, probably due to increasing thickness of the wall as the cell ages.
'The ash content of Ihc sap was found lo bo 4.14±0.0S grains pi-r 100 grains total weight of sap; that of the wall was 19.2± 0.50 grams per 100 grams total weight of wall.
THE PENETRATION OF TRIVALENT AND PENTAVAI.ENT ARSENIC.
Figures 4, 5, and 6 show tho penetration of arsenic from three arsenic compounds: As305 (pentavalent), As,03 (trivalent), and atoxyl (sodium para arsanilate)'(pentavalent). They also show the effect of the presence and ahsence of phosphate buffers in the surrounding solution upon the penetration of arsenic. In obtaining these values, the total amount of arsenic in each of the three com
ponents of the plant—cell wall, protoplasm, and sap—was estimated, and from this the number of micromilligrams of As per gram of fresh substance was calculated. Thus, in order to make the figures for protoplasm comparable with those for sap (as explained previously), the amount of arsenic found in the protoplasm was multiplied by tho factor 1G4, since the sap is about 164 times heavier than protoplasm. In the same way, the figures for the As content of the cell wall were multiplied by 257.
These figures show that by far the greatest amount of arsenic is taken up by the protoplasm. This is of interest in the light of recent studies by Voegtlin, Dyer, and Leonard (6), who have shown that when glutathione is injected into animals in conjunction with 3 amino-4 hydroxyphenyl arsenious oxide (arsenoxide), detoxification of the arsenic occurs. This result suggests that the arsenic unites with the SH group of the glutathione of the protoplasm. The large amount
of arsenic accumulating in the protoplasm of Valonia as compared with that found in the cell wall and sap is in agreement with this explanation. Since the sap contains only a sight amount of organic matter in addition to the salts, it is reasonable to assume that the arsenic present does not exceed the concentration which is in diffusion equilibrium with the protoplasm.
In the writer's experiments on penetration of arsenic into Nilella it was also shown that the wall contained considerably more arsenic than the sap. The wall in this case included the protoplasm, which
could not be satisfactorily disconnected from it as in tlie case of Valonia. The results, therefore, were only qualitative.
Figures 4, 5, and G also show that the pentavalent form of arsenic is taken up and retained in greater amounts by the protoplasm and attains a lower concentration in the sap, although when the trivalent form is used, much less arsenic is found in the protoplasm and about twice as great a relative concentration is found in the sap.
In the acid range, trivalent arsenic does not behave the same as pentavalent, but shows a general low level of penetration. The alkaline range, however, is similar to that of pentavalent arsenic, in that there is an increase in penetration with an increase in alkalinity. The arsenic contents of the sap are given on a separate scale, since it was impossible to represent them accurately on the smaller scale.
In animals, the largest part of pentavalent arsenic is rapidly excreted by the kidney, whereas most of the arsenic in the trivalent