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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 shown 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.

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The modal number of primary double bundles in the region of transition from root to stem structure at the base of the hypocotyl is four in the dimerous and six in the trimerous and hemitrimerous seedling. In the normal seedlings more than four bundles may occur, but in no case have fewer than this number been observed. In the trimerous seedling variation both above and below the mode is found, the numbers ranging from four to eight. On the average the number is from 1.38 to 1.89 bundles higher (or from 30.5 to 47.0% higher) in the trimerous than in the dimerous seedlings.

Intercalary bundles, which are rather uncommon in seedling anatomy in general, occur in from 11 to 46% of the normal seedlings, whereas they are found in only 9 to 29% of the trimerous and in 28 to 43% of hemitrimerous seedlings. The average number of intercalary bundles is also generally higher in the dimerous plantlets.

Considering the total bundle number at the base of the hypocotyl (primary bundles plus intercalary bundles) the trimerous and hemitrimerous seedlings have from 0.77 to 1.91 bundles, or from 14.4 to 46.7%, more than the dimerous seedlings. The differentiation of the dimerous

and trimerous seedlings is conspicuously shown by the frequency distributions of two of the lines shown in diagram 1.

In passing upward from the base of the hypocotyl, each primary bundle pair normally divides into two so that in the central region of the hypocotyl the bundle number is normally twice the number of primary double bundles at the base, plus the intercalary bundles. In many cases the number is somewhat in excess of this, however, showing either that new (intercalary) bundles have appeared or that some of the bundles have become subdivided.

The modal number of bundles in the mid-region of the hypocotyl is eight or ten in dimerous plantlets; in trimerous and hemitrimerous plantlets

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Percentage frequency distributions of total bundles (primary double bundles counted as two) at the base of the hypocotyl in dimerous and trimerous seedlings of two lines. Abscissae represent bundle numbers, ordinates represent percentage frequencies.

it is twelve. On the average the number is from 1.7 to 3.8 bundles higher (or from 15.7 to 47.9% higher) in the trimerous than in the dimerous seedlings. The differentiation of the two classes of seedlings in their vascular anatomy at the level is clearly shown in diagram 2.

The bundles in the mid-region of the epicotyl show in dimerous plantlets a modal number of twelve, whereas in trimerous seedlings it is fifteen. On the average there are from 2.8 to 3.7, or from 23.0 to 30.2%, more bundles in the epicotyl of the trimerous than in the dimerous seedling.

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