with occasional regulation of the current, until the cathode wire had attained saturation, the time required being calculable from the data previously obtained. After the electrolytic current had been interrupted by withdrawing the plug S1 the resistance-measuring circuit was closed by throwing the switch S2 into position 2. The current, derived from the battery B2, which consisted of four large storage cells in parallel, was now regulated by means of the resistance box R2 until the millammeter M2 gave the reading sought. This was from 2 to 4 milliamperes, these values being selected upon the consideration that for the sake of constancy the current should be as small as the precision of the potentiometer would permit. 6.0 I=2.1 2 4 6' 8' 10' 12' 14' 16' 18' 20' 22' 24' 26' 28' Time in Minutes FIG. 2 Dependence of resistance on current By means of the Pohl commutator P the potentiometer was now connected alternately to the potential leads, p and p', coming from the ends of the cathode wire, and to those of the ten-ohm comparison resistance, readings of the fall of potential across the two resistances being taken several times in rapid succession, while the time was in each case noted, in order that the value for the ten-ohm might be calculated by interpolation for the exact moment at which that for the cathode wire was observed. This made it possible to eliminate the error which would otherwise have resulted from the gradual drift in the resistance of the cathode, and hence in the measuring current. A complete experiment consisted of three such series of observations as that which has just been described, obtained at intervals as short as possible. The first and last were taken with a measuring current of ap proximately 4 milliamperes, and the intermediate one with a current of half this intensity. The potentiometer employed was a low-resistance "Type K" instrument by Leeds and Northrup, with which was used a suspended coil galvanometer by the same makers. The latter was adjusted by means of a shunt until its vibrations were aperiodic, and had under these conditions a sensibility more than adequate. Care was taken before the beginning of an experiment to bring the potentiometer battery to such constancy of e. m. f. that errors from this source were negligible, and this constancy was checked at least once in the course of each series of observations by comparisons with a set of standard cells. The ten-ohm comparison resistance used was not a precision coil, so that the resistances found are only relative. Their relative accuracy may be estimated at 0.02% in the least favorable instances. Results. The results of one experiment are recorded in table I, and displayed graphically in figure 2. Before this run the wire had been several times saturated with hydrogen, and immediately before the interruption of electrolysis the current had been maintained over night at a value of 2.3 milliamperes, corresponding to a cathode current density of about 1.4 amperes per square decimeter. In the table below, column 1 shows the measuring current in milliamperes; column 2 the time elapsed from the first reading in seconds; and column 3 the interval between successive observations. The unenclosed numbers in the column headed E10 give the observed fall of potential across the ten-ohm resistance, and the numbers in parentheses are interpolated from the preceding for the time at which E, was read. E, is the observed fall of potential across the cathode wire; and R, is the resistance of the latter calculated from the relation R1 = 10 Ex/(E10). In figure 2 the values of t from table I are plotted as abscissae, and those of R, as ordinates. If the resistance were independent of the measuring current, the values for all three series of the table should lie upon an unbroken curve which is very nearly rectilinear; but it will be seen that the points for the intermediate series, taken with a measuring current only half as great as that used in the two end series, fall far out of line. A diminution of current of 50% has produced a diminution of resistance of some 30%. This result was confirmed by that of an entirely similar experiment in which the changes of current and of resistance were, respectively, fifty and forty-two per cent; and finally by that of an experiment in which the electrolyte was withdrawn from the cell, before the resistance of the wire was observed, and in which the corresponding decrements were fifty and twenty-six per cent. It should be pointed out that only qualitative agreement between the several experiments was to be expected, since the magnitude of the effect is dependent both upon the quantity and upon the condition of the occluded hydrogen at the time of observation. That the results were not materially affected by fluctuations of the measuring current is probably sufficiently attested by the regularity of the observations in each series. The precaution was nevertheless taken of substituting for the palladium wire a nearly equivalent manganin resistance, and conducting with this a similar series of measurements. The resistances found were in this instance constant to the fourth significant figure, showing that with the battery employed, which had a capacity of 240 ampere-hours, and supplied only 4 milliamperes or less, at 2 volts, the variations were wholly negligible. Conclusion. The fact seems to be established by the foregoing experiments that the temporary supplementary conduction exhibited by metals during cathodic occlusion of hydrogen, and for some time thereafter, is not of ordinary metallic character. Strictly regarded, this has been shown only in the case of palladium-hydrogen; but the great similarity previously shown3 to exist in the comportment of palladium, of tantalum and of iron toward hydrogen, and of palladium toward oxygen, makes it probable that the supplementary conduction is of the same character in all cases. The outcome of the experiments is accordingly in conformity with the prediction based upon the first of the three explanations of the conduction which were enumerated above, and lends support to this conception. Upon either of the other two hypotheses the departure from Ohm's law would be difficult to understand. Obviously, it is to be expected that with measuring currents much higher than those here employed the supplementary conduction would approach a limiting value, owing to the inability of ordinary diffusion to maintain the concentration of hydrogen atoms in the regions from which these are driven by the current. Above this limit the dependence of resistance on current would not be observed. Unfortunately the likelihood of being able to test this further prediction experimentally seems small; for it may be seen from the magnitude of the change of resistance in the range studied that its disappearance can be looked for only at very large values of the measuring current; and with such currents the heating effects would render doubtful any results which might be obtained. In concluding, acknowledgment should be made of the excellent assistance rendered in these experiments by Mr. Robert F. Mehl. * The assumption of the existence of molecular hydrogen within the metal seems to be made necessary by certain facts which are not here considered, and the present considerations will not be affected if the quantity of the molecular form is taken to be extremely small, or even zero. 1 Smith, D. P., and Martin, F. H., J. Amer. Chem. Soc., 38, 1916 (2577); Harding, E. A., and Smith, D. P., Ibid., 40, 1918 (1508). Summarized in Trans. Amer. Electrochem. Soc., 34, 1918 (177). 2 Newbery, E., J. Amer. Chem. Soc., 41, 1919 (1887). Harding and Smith, loc. cit. supra. THE VASCULAR ANATOMY OF NORMAL AND VARIANT SEEDLINGS OF PHASEOLUS VULGARIS BY J. ARTHUR HARRIS AND EDMUND W. SINNOTT STATION FOR EXPERIMENTAL EVOLUTION, CARNEGIE INSTITUTION OF WASHINGTON Communicated by C. B. Davenport, November 29, 1920 The investigations here summarized comprise a comparative and biometric study of the gross vascular anatomy of normal and variant seedlings of Phaseolus vulgaris. Three morphological types have been considered, (a) the normal or dimerous seedling with two cotyledons and two primordial leaves, (b) the trimerous seedling with three cotyledons and three primordial leaves, and (c) the hemitrimerous seedling in which there are three cotyledons and two primordial leaves. In normal seedlings, the vascular system of the root is typically tetrarch (with four protoxylem poles), and gives rise in the base of the hypocotyl to four pairs of double bundles which soon form a circle of eight bundles which continue to the cotyledonary node. At this point there is a complex vascular anastomosis. From it two strands are given off to each cotyledon. The remainder of the vascular tissue is reorganized into six strands, each of which typically soon divides into two, the twelve bundles thus formed comprising the vascular system of the epicotyl. The trimerous seedlings typically possess six root poles instead of four, twelve bundles in the hypocotyl instead of eight, and nine primary epicotyledonary bundles instead of six. The nine primary epicotyledonary bundles do not all divide, however, so that the number of bundles in the central region of the epicotyl is variable ranging in general from fourteen to eighteen. In both classes of seedlings, but more frequently in the normal type, additional or intercalary bundles appear in the hypocotyl, either de novo or as a result of division of the primary strands. Four main groups of problems as to the vascular topography of these seedling types have been considered biometrically: First, the number of bundles at different levels in the seedling; second, the variability in bundle number; third, the differentiation in internal structure of seedlings which are externally dimerous, trimerous and hemitrimerous; and fourth, the interrelationship of bundle number in different regions of the seedling. The following table of constants1 summarizes the facts for number and variability of vascular bundles in various regions of the seedling and in the three types of seedlings.2 The constants in this table, and the frequency distributions from which the constants were computed, lead to the following conclusions. |