FURTHER EXPERIMENTS ON PLAGIOTROPISM AND CORRELATIVE INHIBITION

Transcription

1 [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) drew the following conclusion amongst others, that in this species, in which the plagiotropism of the branches isfixedfrom the start, inhibition does not travel up into a branch, though in a plant of the same species with more than one orthotropic leading shoot it can travel from one such shoot up another, just as also it can in seedlings of the common Leguminosae, in which all shoots are orthotropic. His evidence for this conclusion was that when the tip of a branch is cut off lateral buds grow out from the branch a little below the cut and replace the missing tip. These replacement buds are quite distinct from those which grow out normally to form the visibly different branches of the second order, and clearly they can be inhibited, since they were inhibited previously by the tip of the branch. But since they grow out when the tip of the branch is cut off, they are not inhibited by the tip of the main shoot together with the tips of the other branches, or only very little. Nevertheless the tip of the main shoot or of another branch can inhibit these replacement buds if it is grafted on to the end of the decapitated branch. So apparently in this species inhibition can travel down a branch but not up into it. (The mere fact that the branches grow does not show that inhibition cannot travel up them; for so also do two orthotropic shoots if they happen to be equally strong.) In Impatiens roylei the epinastism of the branches, on which their plagiotropism depends, is induced by the tip of the main shoot; and the writer showed recently (1945) that in that species the growth of a branch is normally not inhibited at all by the main shoot and the other branches, but that if a branch is deprived of its developing leaves, then it becomes more nearly orthotropic and is also gradually inhibited. So in that species induction of epinastism and correlative inhibition are clearly alternatives which tend to exclude each other, and it therefore seemed desirable to find out whether the fact that plagiotropic branches are not inhibited is due to an inability of inhibition to travel up into them, as in Araucaria excelsa. Accordingly the following experiment was performed. Seedlings of Impatiens roylei were grown in the open ground in half-shade until the branches of the strongest whorl near the base of each plant, three or four in number, were beginning to elongate their second internodes. Then the buds formed by each of these branches in the axils of its lowest whorl of leaves, several cm. from its base, were selected for measurement, being only 2 or 3 mm. long and from six to nine in number on each plant. Nine well-matched plants were now chosen and were divided into three groups of three. Those of group (a) were left intact, those of group {b) had the tips of the branches which carried the measured buds cut off just beyond them, and those of group (c) had the tips of those branches cut off, and also the tips of all other branches and of the main shoot, so that they were left with mature leaves only. The mean length, in mm., of the measured buds in each plant of the three groups after 17 days of cool weather in June are given in the table. Their

2 Further experiments on plagiotropism and correlative inhibition 255 growth in each of the plants, except in those of group {a) was fairly uniform as the extremes show, so that it seems unnecessary to calculate probable errors. Table. Impatiens roylei. lengths in mm. of measured buds in each plant after 17 days Group (a) Plant (i) 134 Extremes 27 and 7 of 6 buds (2) 12 3 Extremes 19 and 7 of 9 buds (3) 4'7 Extremes 7 and 3 of 9 buds Group (b) Plant (4) 30 8 Extremes 38 and 20 of 6 buds (5) 22 8 Extremes 25 and 17 of 8 buds (6) 18-9 Extremes 25 and 15 of 7 buds Group (c) Plant {7) 34 9 Extremes 43 and 30 of 6 buds (8) 26-9 Extremes 33 and 22 of 6 buds (9) 25-5 Extremes 35 and 20 of 9 buds As the table shows, the buds in groups {b) and (c) grew out practically equally, but in the intact plants of group (a) they grew much less, though they did grow a httle. Since, therefore, in group (i) removal of the tips of the branches carrying the measured buds made these buds grow out much faster than those of the intact plants of group [a), the buds must previously have been inhibited by the tips of the branches carrying them. But further, since these buds grew out rapidly in group (6) and were scarcely made to grow any more rapidly in group (c), by removing in addition the tips of the main shoot and of all the numerous other branches, they cannot have been inhibited appreciably by the tips of the main shoot and of all the other branches together. So it seems to follow that in this species also inhibition can travel down a plagiotropic branch, but not upwards into it, or scarcely at all. Massart's experiment of grafting the tip of a main shoot on to the end of a decapitated branch was not attempted, since with Impatiens roylei it is not needed. For this species does not form vegetative buds of permanently different kinds, as does Araucaria excelsa, and consequently the experiment reported is enough to show that any growing bud at the end of a branch inhibits strongly the axillary buds formed by that branch, whereas all the similar growing buds elsewhere together scarcely inhibit them at all. A similar experiment was performed on nine seedlings of Salvia coccinea growing in pots in a greenhouse, another species in which the epinastism of the branches is induced by the main shoot. On the strongest pair of opposite branches near the base of each plant the two small buds formed by those branches in the axils of their second pair of leaves, several cm. from their bases, were chosen for measurement. Of these two buds, one is on the outer face of the obliquely ascending branch and one on the inner face, and the outer bud always grows the more strongly, so that outer buds must be compared with outer buds, and inner with inner. The period of the experiment was 10 days for the first of the three plants of each group, and 7 days in warmer weather for the others. After these times the lengths in mm. of the outer and inner buds in the plants of the three groups, which are given the same letters as before, were the following: Outer buds group {a) 6-5, 7. 6, (6) 12, II ) Inner buds group [a] ) 2-5, 4. 4, 3-5, 3, 2-5 (b) 1 8, , ± ± ± ± ±0-22.

3 2^6 R- SNOW Thus in Sahia coccinea also comparison of groups (a) and (b) shows that the tip of a branch inhibits strongly the axillary buds formed by that branch, whereas comparison of groups (b) and (c) shows that the tips of the main shoot and of all the other branches together scarcely inhibit them at all. Again, therefore, it seems to follow that inhibition can scarcely travel up into a plagiotropic branch. These results make it necessary to modify a tentative conclusion reached previously (1945). For the previous experiment of depriving a branch of Impatiens roylei of its developing leaves showed clearly that the inhibition of a branch and the induction in it of epinastism are alternatives; and it was concluded that this tended to support the opinion of Miinch (1938) that they are equivalent, which the writer interpreted as meaning that they are different reactions to the same transmitted influence. But the present results have shown that inhibition cannot travel up a plagiotropic branch, or scarcely at all, whereas the influence inducing epinastism, which in Lnpatiens and Salvia is labile, clearly does travel up it. It therefore now seems more probable that these are two different transmitted influences after all, as is indeed quite possible if inhibition and induction of plagiotropism are indirect processes, in spite of the fact that the first stage in each is the same, being the secretion of auxin by the tip of the main shoot. So, apparently, the situation is that when the branches possess their developing leaves and so have abundant auxin, inhibition cannot travel up into them and the influence inducing epinastism does do so, whereas when they are without these leaves inhibition does travel up them and the influence inducing epinastism either cannot travel up into them or is not efltective when it gets there. In the previous paper (1945, p. 112) another experiment was reported in which a branch of Impatiens roylei was decapitated and so deprived of its developing leaves and was then given a cap of lanoline containing hetero-auxin at i in 350. The result was that it remained dorso-convex like an intact branch, whereas a similar branch similarly treated but without hetero-auxin curved, as usual, dorso-concave. But since this was done on one pair of branches only, the experiment has now been repeated as follows. In each of three seedlings of Impatiens roylei two similar branches of a lower whorl, a few cm. long, had their tips cut off, and one of them was capped with lanoline containing hetero-auxin at i in 350 and the other with vaseline. In 8 days of cool weather the branches curved so that the inclinations of their ends were changed by the following angles: in the branches with hetero-auxin, 20 down, 7 down and 5 down; in those without hetero-auxin, 15 up, 12 up and 10 up. Thus again hetero-auxin applied to the ends of decapitated branches acted instead of their young leaves in keeping them epinastic, though it did not keep them growing at the normal rate. It may at first seem surprising that, in this species at least, hetero-auxin can act instead of developing leaves in keeping a branch epinastic, either when applied to the end of the branch if the branch has been decapitated, or, as was shown previously (1945), when applied to the main shoot if that has been decapitated. For this may make it seem that the hetero-auxin acts differently in a branch from what it does in a main shoot, which itself remains orthotropic. But it needs to be remembered that the hetero-auxin or natural auxin supplied from the end of a branch does not directly make the branch epinastic, but only makes it liable to be made or kept epinastic by the action of the main shoot tip. For if the main shoot tip is removed, the branches soon curve up and replace it. But the main shoot itself is dominant, and is not liable to correlative influences from other

4 Further experiments on plagiotropism and correlative inhibition 257 shoots so long as it is dominant; and it therefore remains orthotropic although richly supplied with auxin. Indeed an abundant supply of auxin from its tip is the very thing which makes a shoot dominant, as is clear from earlier experiments by the writer (1931). Although in the species so far investigated it has been shown that inhibition cannot travel up into a plagiotropic branch, or scarcely at all, yet it can penetrate a plagiotropic lateral bud. For in Impatiens and Salvia the lateral buds on a branch have been shown to be inhibited by the tip of that branch, and so also, as Massart (1924) showed, are the replacement buds on a branch of Araucaria excelsa, though not the buds destined to form the branches of the second order. This difference may perhaps be due simply to the difference in the length of the upward path, the inhibition being able to travel upwards for the short distance into a little lateral bud. In any case, since inhibition does travel up into some plagiotropic buds, the freedom from inhibition, even at the earliest stages, of those other plagiotropic buds which develop into shoots of lower order than the shoots carrying them can hardly be due to an inability of inhibition to travel up into them. Such buds are formed by Araucaria excelsa and many tropical species, as was mentioned previously (1945, p. 115). Their freedom from inhibition seems to be a different phenomenon and probably connected with precocity. For it is apparently the rule that free buds are more precocious than inhibited replacement buds of the same species (see Sandt, 1925, p. 90). When the free buds have grown out into branches, then they may be expected to remain free from inhibition, since inhibition can scarcely travel up a plagiotropic branch. This opportunity may be taken to mention that epinastism may be to some degree induced by the tip of the main shoot in leaves as well as in lateral shoots. For in Stachys sitvatica it was several times noticed that a pair of leaves which had just emerged from the terminal bud but were still vertical, descended much less rapidly than usual to their normal positions if the terminal bud was cut off just above them. SUMMARY 1. Correlative inhibition cannot travel up into plagiotropic branches of Impatiens roylei and Salvia coccinea, or scarcely at all, though it does travel down them, as was shown by Massart (1924) for Araucaria excelsa also. 2. In Impatiens roylei hetero-auxin in lanoline can act instead of developing leaves in keeping a branch epinastic and so plagiotropic, either when applied to the end of the branch if the branch has been decapitated, or when applied to the end of the main shoot if that has been decapitated. 3. In the light of these results the fact, shown previously (1945), that induction of epinastism and correlative inhibition tend to be alternative is further discussed. The freedom from inhibition of certain plagiotropic buds in other species is also discussed. REFERENCES MASSAKT, J. (1924). La co-operation et le conflit des reflexes...ehez Araucaria excetsa. Mem. Acad. R. Belg. Cl. Sci. s^rie 2, S, 1313.,. D, SA Q MONCH, E. (1938). Untersuchungen iiber die Harmonie der Baumgestalt. J. wtss. Bot. 86, 581. SANDT W. (1925). Zur Kenntnis der Beiknospen. Jena. o r. o SNOW R. (1931). Experiments on growth and inhibition. Part 2. Proc. Roy. Soc. B, 108, 305. SNOW', R. (194s). Plagiotropism and correlative inhibition. New Phytot. ^i, 110.

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