[ A WOUND SUBSTANCE RETARDING GROWTH IN ROOTS BY SIR FREDERICK KEEBLE, C.B.E., Sc.D., F.R.S., M. G. NELSON, M.A., AND R. SNOW, M.A.

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1 [ A WOUND SUBSTANCE RETARDING GROWTH IN ROOTS BY SIR FREDERICK KEEBLE, C.B.E., Sc.D., F.R.S., M. G. NELSON, M.A., AND R. SNOW, M.A. (From the Department of Botany, Oxford) I T has become well known through the experiments of Haberlandt and others (l, 3) that substances produced at the surfaces of wounds in plant tissues often increase the divisions of the underlying cells, and that these substances can be largely removed by washing. In the present note we report experiments which show, as we consider, another effect of wound substances removable by washing namely, a retardation in the growth of decapitated roots. Our experiments were performed on the main roots of young seedhngs of Zea Mays, and of Pisum sativum, which were germinated in sawdust and kept both before and during the experiments in an electric thermostat at a constant temperature of 20 C. The plan was to compare the growth of roots which were decapitated and then washed for 15 Or 30 minutes, with that of roots which were similarly decapitated but not washed afterwards. The latter were, however, always immersed in water before decapitation, for the same time and in the same conditions, in order that they might have an equal opportunity of absorbing water. Since individual roots vary greatly in rate of growth, it is extremely important to select for comparison sets of roots that are as similar as possible. We, therefore, adopted the following method throughout. Large numbers of seedlings were grown, in batches of the same age, and the rates of growth of their roots were measured during a preliminary period of from 16 to 24 hours, or more often during two prehminary periods of about 7 and 18 hours. Pairs of seedlings were then selected which had grown at almost exactly the same rate during these periods, and were also of approximately the same length. The members of each pair were then separated into two sets, to be operated upon as experiments and controls. The method was laborious, and it also made it necessary to reject very many of the seedlings grown; but we found that it increased very greatly the consistency of the results. The underlying assumption was, naturally, that of roots

2 290 SIR FREDERICK KEEBLE, M. G. NELSON AND R. SNOW that had been matched in this way were much more likely to continue growing at the same rate than taken at random. In order to wash the roots, we fixed the seedlings with their roots vertical by pinning the seeds to strips of cork at the bottom of glass vessels. The vessels were then kept filled with running tap-water, so that the entire seedlings were submerged. After the washing, the vessels were emptied of water, except for a shallow layer at the bottom, and were then closed with a lid, so tbat the seedhngs were left with their roots in saturated air. If it was so required, they were easily rearranged with the roots horizontal. Strips of filter-paper were placed with one end submerged and the other end wrapped round the seeds, so as to supply water through them to the roots: for, as is well known, roots in damp air soon begin to slow down in growth, unless they are somehow kept supplied with water. With this arrangement, the roots grew rapidly and remained perfectly healthy for 48 hours. In the four following tables we give the results of three different experiments with maize seedlings, and of one with peas. In each table there are included the results obtained with several separate series of seedlings, and the figures represent the mean growth of the decapitated and washed roots of each series after various times, expressed as a percentage of the growth of the decapitated but unwashed controls with which they were paired. The roots were washed for 15 minutes in running tap-water, except those of Series 5 of Table I and Series i of Table III, which were washed for 30 minutes in still water. Expt. I. (Table I.) ZeaMays. Growth of roots decapitated at I mm. from end of root-cap and washed, expressed as a percentage of tbe growth of similarly decapitated but unwashed roots that were paired with them. The roots were kept vertical. TABLE I Growth in No. oi first 5 to 6J hours 15 J to 17 hours 6J to 8 hours Series i ,, ,, ,, 4, ioo-o Total 46 Mean Expt. 2. (Table II.) Zea Mays. This experiment was similar to Expt. I, except that the roots were arranged horizontal. The results are expressed similarly.

3 A Wound Substance retarding Growth in Roots 291 Series i,, No. of II 10 6 Total 52 TABLE II Growth in first 5 to 7 hours Mean to 17 hours I2I-O to 7 hours IOO-O ^. (Table III.) ZeaMays. As Expt. i, except that the roots -were decapitated at 0-5 mm. from end of root-cap. Results expressed similarly. TABLE III Series i,, No. of Total 39 Growth in first 6 hours Mean hours III Expt. 4. (Table IV.) Pistim sativum. The roots were decapitated at I mm. from end of root-cap and arranged vertical, as in Expt. i Series i,. 2 No. of 15 II TABLE IV Growth in first 5j hours no J hours no III 6 to 8 hours Total 26 Mean From the first three tables it can be seen that in all the 13 series of maize roots except one (Series 3 of Table II) the washed roots grew the faster on the whole, and further that in eight of the series they grew faster in every period of measurement. It does not appear that it made any difference to the results whether the of roots were vertical or horizontal. Table IV shows that similar results were obtained with roots of Pisum sativum in the first two periods of measurement, though in the third period (after 22 hours) the washed roots fell slightly behind the controls in rate of growth. The results, when considered together, are so consistent that it seems certain that washing for 15 or 30 minutes did accelerate the growth of the decapitated roots, and further that the effect lasted for 22 hours or more. We interpret this effect as showing that wound substances which PHVT. XXIX. 4 ^

4 292 SIR FREDERICK KEEBLE, M. G. NELSON AND R. SNOW retard growth are produced at the cut surface of the root, and that these are largely removed by washing. It might indeed be suggested that the washed roots grew faster because their cut surfaces enabled them to absorb more water than the controls, although the controls, as already stated, were immersed for the same time before decapitation. But since the root-cap, in Zea Mays, is about 0-5 mm. long, the cuts made in Expts. i and 2 at i-o mm. from the end must have passed only 0-5 mm. behind the vegetative apex, in a region where the differentiation of vascular tissue is only just beginning. Furthermore, in Expt. 3, the cuts were made at only 0-5 mm. from the end, and must therefore only just have grazed the vegetative apex. When therefore it is considered that all the seedlings, with the whole absorbing zones of their roots, were completely submerged during the period of washing, and also that afterwards water was continually supplied to the seeds in the manner already explained, it can hardly be supposed that the very small cut surfaces at the apices of the roots increased the absorption of water appreciably. It seemed possible that the effect of wound substances on the rate of cell-division in the root might be different from their effect on growth, and that they might even accelerate cell-division, as they do in other organs. We, therefore, attempted to determine this point by noticing whether these substances affect the time taken by a decapitated root to regenerate a new tip. Eor this purpose, we decapitated roots of seedlings of Vicia Faha, but found that the time needed for regeneration of new tips was not appreciably altered either by washing the roots for 15 minutes on the one hand, or, on the other hand by covering their cut surfaces with a meal made by crushing several root-tips in a little water. We also found that roots of Vicia Faha regenerate new tips perfectly well, even when they are decapitated under water, transferred qviickly to running tap-water, and then kept submerged in running water for the whole time. It, therefore, appears that wound substances are not necessary for the regeneration of the root-tip, and there is so far no evidence that they even alter the rate at which it takes place. Nemec had previously come on other grounds to a similar conclusion as to the effect of the wound ((2), p. 230). It is, however, quite possible that the wound substances derived from the root-tip may be found to accelerate celldivisions in some of the other parts of tbe plant which have been found to be strongly affected in this way by their own wound substances.

5 A Wound Substance retarding Growth in Roots 293 SUMMARY 1. Roots of seedhngs of Zea Mays and Pisum sativum that are decapitated and then washed for 15 or 30 minutes grow faster, during the next 22 hours or more, than similar decapitated roots that have been submerged for the same time before decapitation instead of after it, 2. It is concluded that substances retarding growth are formed at the wound, and that they are largely removed when the roots are washed afterwards. REEERENCES (1) HABERLANDT, G, Uber Auslosung von Zellteilungen durch Wundhormone. Sitzungsbey. d. Pretiss. Alt. d. Wiss. 8, (2) NEMEC, B. Stiidien iiber die Regeneration. Berlin, (3) WEHNELT, B. Untersuchungen (iber das Wundhormon der Pflanzen. Jahrb. /. wiss. Bot. 66,

FURTHER EXPERIMENTS ON PLAGIOTROPISM AND CORRELATIVE INHIBITION

[254] FURTHER EXPERIMENTS ON PLAGIOTROPISM AND CORRELATIVE INHIBITION BY R. SNOW Fellozv of Magdalen College, Oxford From his excellent study of correlative inhibition in Araucaria excelsa Massart (1924)