ON THE INTERPRETATION OF GEOSTROPHIC AND AUXIN TORSIONS

Transcription

1 JULY 1950 VOL. 49, No. 2 ON THE INTERPRETATION OF GEOSTROPHIC AND AUXIN TORSIONS BY R. SNOW Fellow of Magdalen College, Oxford (With 3 figures in the text) I. INTRODUCTION The purpose of this paper is to analyse and interpret, so far as is possible, the natural geostrophic torsions of growing organs, and also those caused by applied auxin, in the light of evidence presented previously (1942, 1945, 1946, 1947) or to be presented here. The original purpose of these studies was to analyse only the process of movement; but after the discovery of the torsions which are caused in petioles, stems and roots by hetero-auxin applied to them on one fiank when horizontal or inchned (1945, 1947), it can hardly be doubted that the natural geostrophic torsions of petioles and dorsiventral stems are also introduced by changes in the auxin conditions within them: for the resemblances between these two kinds of torsions were pointed out previously (1945, 1947) and will appear again in the present paper. So there has now emerged the new question, what are the changes in the internal auxin conditions which lead to these torsions? And it will be simpler to discuss these auxin changes before discussing the growth responses, since they are an earlier stage of the whole process. The term strophism will correspond with tropism, torsion with curvature, and strophic or geostrophic torsion with tropic or geotropic curvature. Epinastism will be used for the tendency to make an epinastic curvature or epinasty, in agreement with Miinch (1938, p. 646). In discussing the auxin conditions, it will be accepted as being now a well-established theory that petioles and dorsiventral stems keep their normal inclination in relation to gravity through a balance between epinastism and negative geotropism. Evidence for this, both old and new, is well set out by Rawitscher (1932), and some further good evidence is given by Miinch (1937, 1938). It is, however, a great handicap that the nature of epinastism is still not known with certainty. Experiments by Uyldert (1931) on shoots of Tradescantia do indeed make it very probable that epinastism is based, as might be expected, on a tendency somehow to raise the concentration of auxin in the dorsal half above that of the ventral half. But unfortunately she was handicapped by being unable to extract auxin by diffusion from this species, though she could cause curvatures by applying auxin, so that the evidence is indirect. For the same reason it is still doubtful how this rise in the concentration of auxin is brought about. Uyldert's experiments 23, 24 and 25 make it seem that in an epinastic curvature down from the vertical the concentration of auxin is greater in the dorsal half because this half conducts auxin longitudinally towards the base whereas the ventral half fails to do so. But the geonegative curvatures up from the horizontal (Exp. 26) appear to be due to a transverse downward flow or redistribution of auxin, such as occurs in other plants, in the basal part of each internode. This is how Uyldert herself interprets her experiments, unless the writer 10 New Phytol. 49-2

2 146 R. SNOW misunderstands her, though in her summary (p. 66) she does not bring out this distinction so clearly. But there is much that is still puzzling for instance, evidence (Exp. 26) indicating that in horizontal, though not in vertical, shoots the dorsal and ventral halves conduct auxin longitudinally equally well, which is difficult to reconcile with the evidence from the same genus that epinastism is equally strong at all angles (Rawitscher, 1932). So investigations on other plants from which auxin can be extracted are badly needed. 2. THE AUXIN CONDITIONS LEADING TO TORSIONS It will be convenient to distinguish and compare three groups of torsions. The first group is the torsions of orthotropic stems, coleoptiles and roots caused by applying auxm to them on one flank when horizontal (1947). These torsions bring the treated flank towards the top (see figs I, 2, 3). The second group is the torsions made by petioles and plagiotropic stems that are similarly treated with auxin on one flank when in their equilibrium position (1945). These also bring the treated flank towards the top. All these torsions caused by applied auxin will be called auxin torsions. The third group is the natural geostrophic torsions which are made by petioles and plagiotropic stems when turned on to their flanks and which bring them back to the correct orientation. It may be noted that in all the experiments the light came mainly from overhead, and so acted in the same sense as the stimulus of gravity. It is common to all these torsions that they are set going when a face of the organ which either is treated artificially with auxin or tends to be promoted in growth by epinastism, is not opposite to another face towards which auxin tends to be accumulated through the action of a transverse tropic stimulus of gravity or presumably of light. The torsion moves the former of these faces by the shorter way round towards the position opposite to the latter face towards the top, that is, when the stimulus is gravity. From this rule it follows that the effects of epinastism and of artificial application of auxin are not the same as those of the transverse stimuli of gravity and light. They may, indeed, tend equally strongly to cause curvatures, so that epinastism or application of auxin can be balanced against a transverse stimulus of gravity when the two are acting in opposition; hut the torsions which result when they are not exactly in opposition and which take place always in the same direction in relation to the stimulus of gravity show that the effects of the factors of these two classes must be somehow different. It has, indeed, been shown (1947) that in epicotyls of Phaseolus two similar stimulations by gravity in planes at right angles do not cause torsion. So the effects of epinastism and of artificial application of auxin in relation to the torsions are both somehow different from the effects of transverse tropic stimuli of gravity and presumably of light, but apparently similar to each other. For greater accuracy it must be added that when epinastism and applied auxin are at work in different planes, as in a petiole treated with auxin on one Rank, there appears to be a resultant of their combined effects in an intermediate plane; and similarly two tropic stimuli in different planes would presumably form a resultant in an intermediate plane. So torsional equilibrium is reached when the resultants of the factors of the two classes are opposite. Possibly a natural supply of auxin to one side is a third factor which may be added to the class comprising application of auxin to one side and epinastism. For some young leaves of Spiraea arborea with the leaflets removed from one side were found to twist so as

3 On the interpretation of geostrophic and auxin torsions 147 to raise the side with the remaining leaflets, though the torsions usually disappeared some days later. But the results were rather erratic, and further study is needed. It seems at first tempting to suggest further that in the equilibrium orientation, as described above, there is a balanced downward transverse flow of auxin due to the stimulus of gravity; but that in other orientations the transverse flow is unbalanced, being greater in one half of the organ than in the other (Figs, i, 2), and that this unbalanced transverse flow is the next stage in the process leading to a torsion. Indeed, in the auxin torsions of Fig. 1. Fig. 2. Fig- 3- Figs. 1-3 illustrate the suggestion of an unbalanced downward flow of auxin due to the stimulus of gravity. The longer straight arrows indicate stronger downward flows; the curved arrows show the direction of torsion. Fig. I. Transection of a stem laid horizontal and treated with auxin paste (cross-hatched) on one side. Fig. 2. The same of a petiole turned on to its flank. Fig. 3. The same of a petiole treated with auxin paste on one side. orthotropic organs, which are the simplest to interpret, a flow of this kind seems quite probable, since it is known that in such organs transverse stimulation by gravity or light causes the auxin to flow towards the lower face or the shaded face. When, therefore, such an organ is treated with auxin on one flank while horizontal, it may be expected that the transverse downward flow due to the stimulus of gravity will be greater in the half in which the concentration of auxin has thus been raised (Fig. i). Also in the natural

4 148 R. SNOW geostrophic torsions made by petioles when turned on to their flanks it seems possible that the stimulus of gravity may cause an asymmetric downward transverse flow of auxin. For in such petioles the epinastism is free to raise the concentration of auxin in the dorsal half, being no longer opposed by the stimulus of gravity, so that again the stimulus of gravity may cause a greater transverse flow of auxin in the half with the higher concentration that is, in the dorsal half (Fig. 2). This suggestion does imply that the auxin changes due to epinastism must differ from those due to the stimulus of gravity, but it has been explained already that one must in any case conclude that the effects of epinastism differ somehow from those of the stimulus of gravity. However, it is more difficult to explain in a similar way the auxin torsions of petioles, since these are caused by applying auxin to one flank while the petiole is inclined at its normal angle. So it is doubtful whether the stimulus of gravity is then inducing a transverse downward flow of auxin, since at the normal angle on the accepted theory negative geotropism and epinastism are just balanced against each other. Also the following experiment makes it seem unlikely that there is much downward flow of auxin in these petioles when at their normal angle. If one applies equal streaks of auxin paste to bothflanksof a young petiole, their tendencies to cause torsion will of course be opposed and cancel out; but a transverse downward flow of the applied auxin would be revealed by an upward curvature. Accordingly streaks of lanoline containing hetero-auxin at concentration i in 300 were applied to both flanks of the distal parts of young petioles of the first pair of leaves of Phaseolus multifioriis, similar to the streaks which cause strong auxin torsions when applied to one flank, and in similar conditions. It was found that when care was taken to place the streaks accurately along the mid-line of each flank, they caused very little curvature, if any. The treated petioles did, indeed, make upward curvatures in the next day or two which reached 20, though they diminished later; but the untreated petioles of the other leaves of the same pairs also made upward curvatures up to 12, and the differences were not signiflcant. In spite of this, the stimulus of gravity is necessary for the auxin torsions of these Phaseolus petioles, since they do not twist if the plants are rotated on the horizontal axis of the clinostat, as the following experiments show. In each of three Phaseolus seedlings the petiole, only 2 or 3 cm. long, of one of the two flrst leaves was treated on one flank in the distal part with anauxin paste of i in 300 which was continued onto the distal pulvinus and the pots were then rotated on the horizontal axis of a clinostat in a greenhouse, the stems being in the line of the axis. After 22i hr. two of the three treated petioles were still not twisted, and the third had twisted only 15, so as to move the paste towards the dorsum. The three had curved -90, -100 and -25 (away, that is, from the applied auxin). The pots were now taken off and placed upright, and after only about 3 hr. more the petioles had twisted 60, 80 and 80, so as to raise the paste. In a fourth seedling after 6 hr. on the clinostat the treated petiole had not twisted but had curved -90, and after another 18 hr. with the pot upright it had twisted 70 so as to raise the paste. So without the stimulus of gravity the petioles did not twist, or scarcely, but with it they afterwards twisted especially rapidly. The directions of the auxin torsions of these petioles are also determined by the stimulus of gravity, as was shown previously (1945, p. 79), since even in inverted plants the petioles twist in the direction which raises the paste, although this is then the opposite direction

5 On the interpretation of geostrophic and auxin torsions 149 in relation to the structure of the petiole and to its epinastism. Thus these auxin torsions are really also geostrophic. So it comes to this, that the stimulus of gravity is necessary for the auxin torsions of these petioles and determines their directions, although in respect of its tendency to induce curvatures and apparently of its tendency to cause a transverse flow of auxin it is fully counteracted by epinastism. However, it is still just possible that the epinastism may counteract the negative geotropism not by stopping the transverse downward flow, but in some other way. This is, indeed, suggested for Tradescantia shoots by the experiments of Uyldert already mentioned, but, on the other hand, the experiment with petioles of Phaseolus treated with auxin on both sides reported above makes it more probable that in them the transverse downward flow is indeed stopped at the normal angle, or nearly so. This is about as far as it seems worth while to go until such time as the nature of epinastism may be cleared up; and meanwhile it must remain only a suggestion worth testing that an unbalanced transverse flow of auxin induced by the tropic stimulus is the second stage in the process leading to a torsion the stage which results from the condition of disequilibrium described above. Nothing of all this is intended to apply to the torsions of pulvini. For the distal pulvini of Phaseolus leaves, though they make good geostrophic torsions, have repeatedly been found not to twist when treated with auxin on one flank, or not appreciably, but only to curve away. 3. SOME FURTHER POINTS CONCERNING AUXIN RELATIONS There are some further points which must now be mentioned and discussed, although they lie rather apart from the main argument. Since the petioles of Phaseolus make practically no auxin torsions on the clinostat, it is clear that in these petioles the auxin torsions depend on the stimulus of gravity; and the interpretation of this conclusion has been discussed. But the young petioles of sunflowers do make some auxin torsion on the clinostat. In young sunflower seedlings standing upright the young petioles, only i cm. long or less, of leaves only just expanded were found to make good auxin torsions when treated on one flank with a streak of hetero-auxin paste of concentration i in 300. Thus in 6 hr. flve such petioles twisted 50, 50, 50, 40 and 40 so as to raise the paste. In seedlings rotated on the horizontal axis of the clinostat, with their stems in the line of the axis, flve similar petioles similarly treated twisted, in periods of from 6^ to 8 hr., o, 25, 30, 20 and 20, in the same direction relatively to their structure. The last three of these petioles belonged to seedlings which had been rotated for 23 hr. beforehand. Thus on the clinostat the petioles made torsions which were weaker but in the same direction relatively to their structure as in the upright seedlings. This suggested that the stronger auxin torsions of the petioles of upright seedlings might be due to a combination of a tendency based on their structure to move the treated flank towards the dorsum with another tendency, similar to that in Phaseolus, to raise it to the top. So in order to test this suggestion, similar petioles of other seedlings were similarly treated with the paste on one flank, and the pots containing the seedlings were inverted, the blades of the treated leaves being supported at the start on flat blocks to keep them accurately horizontal at the start, as in previous experiments with Phaseolus (see 1945, p. 79); for in inverted plants a torsion tending to move the treated flank towards the dorsum will be opposite in direction to a torsion tending to move it towards the top.

6 150 R. SNOW Four petioles belonging to two seedlings were thus treated. One of them twisted so as to raise the auxin paste 50 in 23 hr., and then changed no more. Two twisted so as to lower the paste 30 and 25 in the first 24 hr.; but then they reversed their torsions and one of them twisted so as to raise the paste to 50 above the horizontal and the other so as to raise it to the top. The fourth twisted so as to lower the paste to the bottom and just beyond it (100 ) in 24 hr., and continued twisting so as to raise it up on the other side. These varying results, with their clear indications of conflicting tendencies, can hardly leave any doubt that the sunflower petioles do, indeed, tend both to twist so as to raise a flank treated with auxin paste to the top and also to twist so as to move that flank towards the dorsum. In the upright seedlings the two tendencies combine, in the inverted seedlings they conflict, and on the clinostat the second acts alone. So it is a lucky chance that the Fhaseolus petioles make auxin torsions that are directed only in relation to the stimulus of gravity, and so are simpler and more instructive. The leafy cotyledons of radish seedlings also made good auxin torsions reaching 50 or 60 in 24 hr., but were not tested on the clinostat. Some other points which seem worth recording are concerned with the part played in the torsions due to applied auxin by the natural auxin coming from the apical region. This question was studied in coleoptiles, since it is known that when these are decapitated their production of auxin is greatly diminished, and that if the seeds are cut ofl^ from the seedlings, the production of auxin by the coleoptiles remains very low (Skoog, 1937).* Oat seedlings grown in the dark were arranged in red light with the coleoptiles horizontal, and some were left entire, while others, which had had the seeds removed, usually from 18 to 24 hr. previously, now had the apical 5 mm. of the coleoptiles cut off. The coleoptiles were given streaks of lanoline containing various concentrations of hetero-auxin along one of their narrow sides, which were in the horizontal plane. The streaks reached as far as the apical ends of the decapitated coleoptiles and to the corresponding level in the others. With a concentration of i in 600, ten entire coleoptiles all made strong torsions ranging from 60 to 90 or even beyond, but fourteen decapitated coleoptiles made practically no torsions, or only a few very feeble ones. With I in 2400, on the other hand, four entire coleoptiles made?io torsions, but eleven out of thirteen decapitated coleoptiles made torsions with a mean of 46. All these torsions were in such a direction as to raise the treated flank. With an intermediate concentration of i in 1200 the results in both groups were erratic. So the best concentration for the decapitated coleoptiles was several times lower than for intact coleoptiles, which were also found previously (1947) to make strong torsions with a concentration of i in 400. It seems, therefore, that the concentration of applied auxin needs to be adjusted rather exactly in order to cause good torsions, perhaps because its effect needs to be about as strong as the geotropism, or only a little stronger. The geotropism was, of course, much weakened by the decapitation and removal of seed. 4. THE ANALYSIS OF THE RESPONSES IN THE TORSIONS The writer's original purpose was only to discover what is the primary response in a strophic torsion. It was pointed out (1942, 1945, p. 80) that according to Schwendener & Krabbe( 1892) the primary response is a change in the direction oi growth, such that the peripheral cells, which previously were elongating in the direction of the long axis of the organ, * Skoog's seedlings were, however, rooted in water, but the writer's in sawdust and sand.

7 On the interpretation of geostrophic and auxin torsions 151 now elongate obliquely in the direction of a helix winding round it, the sense of the helix depending on the direction from which the stimulus is coming. But on Rawitscher's 'transverse components' theory (1932) the primary response is a change in the distribution of growth, the growth concerned being the transverse growth and the elongation of cells in a helical direction being only a consequence. By the distribution of growth is meant the relative rates of growth in different parts. If the primary change is in the direction of growth it is to he expected that an organ flxed at its distal end and set free at its basal end will twist in response to a strophic stimulus in the opposite direction to the normal; but if the primary change is in the distribution of transverse growth, then it will twist in the same direction as the normal, as was fully explained previously (1942). There is also a third possibility that the primary change is in the distribution (not direction) of longitudinal growth, as it is in the tropisms; thus, for example, the primary change might be the setting up of two successive longitudinal curvatures in different planes, or in a dorsiventral organ of one lateral curvature only. If this were so, then again an organ fixed distally would twist in response to a stimulus in the normal direction. So the experiments on the direction of torsion of organs flxed distally really serve to distinguish between a primary change in the direction of growth, and one in the distribution of growth, no matter whether that growth be transverse or longitudinal. It was found (1942, 1945, 1946) that petioles (not including pulvini) of many species that were tested twisted in the normal or positive direction when flxed distally and arranged on their flanks to be stimulated by gravity in the direction, that is, which hrought the correct face to the top again. Some apparent exceptions were reported in the second of these papers (1945), but these were afterwards found to have been errors due to an unsound method (1946). At present the petioles oi Phaseolus are the only ones of those tested for which the direction of torsion is still doubtful. For in that species the petioles of leaves flxed distally and cut off at the base have not been found to make strong or consistent torsions, but only feeble ones, of which most, though not all, are negative. The Phaseolus leaves are especially difficult to keep turgid when cut off and flxed distally. To these results with petioles there can now be added those of the following experiments with young stemsflxeddistally. Eight young sunflower seedlings with hypocotyls only a few centimetres long were cut off at the base of the hypocotyl and laid horizontal with the cotyledons in the horizontal plane. They were flxed distally with a needle passing through the cotyledons and into a block of plasticine, and took up water through the lower parts of the cotyledons, which were just submerged and had their edges cut off. The hypocotyls were each given a thin streak of lanoline containing hetero-auxin at i in 300 or i in 600 on one flank in the distal part. After 6 or 9 hr. seven of the hypocotyls had twisted their free basal ends from + 60 to + 80 (that is, so as to raise the treated flank) and the eighth had twisted +25. Also four young radish seedlings were treated similarly with a paste of i in 300, and after 23 hr. they had twisted by rotating their free basal ends +55, +60, + 55 and So the auxin torsions of these young stems flxed distally were also in the same direction as when they were fixed at the base. This strengthens the evidence that the torsions carried out by growth depend on changes in the distribution, not the direction, of growth, and it also supports the view that the auxin torsions are similar in nature to the strophic torsions of petioles.

8 152 R. SNOW There remains the question whether it is changes in the distribution of the transverse or the longitudinal growth which cause the torsions. The latter might lead to a torsion by causing a single lateral curvature in a dorsiventral organ or two successive curvatures in different planes in a radial organ, and Rawitscher (1932) has done a valuable service by fully discussing these matters. A single lateral curvature, as he makes clear, will cause a torsion in a dorsiventral organ if its dorsal and ventral parts resist the force causing the curvature unequally. Similarly, in a radial organ two successive curvatures in planes at right angles might cause a torsion, since after the first curvature the organ as a whole is no longer radially symmetric, and its convex and concave sides may resist the second curvature unequally. Nevertheless Rawitscher concludes that actually the strophic torsions are not caused in these ways, since some organs, such as the petioles of Wistaria, twist with practically no curvature at all. The writer has previously given reasons for thinking that in his experiments the torsions did not result from curvatures (1942, p. 5; 1947, p. 3), but it seems desirable to discuss this question again with further evidence. It was shown previously (1947, p. 3) that in the epicotyls of Phaseolus two successive geotropic curvatures in planes at right angles do not cause any torsion. In the auxin torsions of these organs the geotropic curvature and the auxin curvatures are simultaneous, and for that reason even less likely to cause a torsion. Indeed they could not do so, unless the forces causing these two curvatures were in some way different, which is a very difficult conception. Also other reasons were given for thinking that the auxin torsions did not result from the curvatures (1947). It is even more difficult to explain the torsions of petioles as resulting from curvatures for the following reason. The petioles which make auxin torsions are inclined at their normal angle when the auxin is applied to them, as was pointed out and discussed above, and consequently the only appreciable curvature which they make is the lateral curvature away from the auxin paste. Now if their dorsal and ventral parts were unequally strong and resisted the lateral force unequally, a torsion would result. But it has been shown that on the clinostat Phaseolus petioles make no auxin torsions, even when they curve as much as 90 away from the auxin paste. So their dorsal and ventral parts do not resist a lateral force unequally, and the auxin torsions of these petioles cannot be caused by their lateral auxin curvatures. For the same reason, that their dorsal and ventral parts do not resist unequally, the geostrophic torsions which these petioles make when arranged on their flanks cannot result from a geonegative curvature. Previously it was found that when petioles were made to curve laterally by being pressed with a rod, those of most of the species tested did twist in consequence, but in the direction in relation to the curvature which was opposite to that of a strophic or auxin torsion (1942, p. 6; 1945, p. 79), and the petiole of Phaseolus was reported to do this slightly. But it is difficult to make this method of testing convincing unless the resulting torsion is fairly large, and so for Phaseolus the experiments with the clinostat reported here which show no torsion resulting from curvature provide much better evidence. Still other evidences showing that in petioles the torsions do not result from the lateral curvatures are the following. First, in inverted plants of Phaseolus the auxin torsion is in the opposite direction in relation to the lateral curvature. Secondly, if the auxin torsions of petioles resulted from the lateral curvatures, they should continue indefinitely instead of stopping when the flank treated with auxin paste has reached, or nearly reached, the top, as actually they do. It seems, therefore, impossible that strophic and auxin torsions can result from curva-

9 On the interpretation of geostrophic and auxi?i torsions 153 tures, and it is difficult to think of any way except by means of curvatures in which a change in the distribution of longitudinal growth could lead to a torsion. So it must be concluded that these torsions in growing organs are caused by a change of some kind in the distribution of transverse growth. This conclusion has admittedly been reached by the rather insecure method of excluding other possibilities, but it is more positively supported by the fact that the auxin torsions of roots are in the direction that raises the treated flank, like those of stems and petioles (1947). although their growth in length is retarded by auxin, except in very low concentrations much lower than those used. For it is known that the transverse growth of roots, like that of stems, is promoted by auxin in about the same concentrations in which it retards growth in length of roots; and the writer has found that if a ring of lanoline containing hetero-auxin at i in 6000 is placed around a young main root of Vicia Faba or Phaseolus very close behind the apex, the cortex of the zone covered by the paste grows distinctly more than the normal in thickness, so that it appears swollen after a day or two. This is the same concentration which previously caused good auxin torsions, raising the treated flank in roots of Vicia Faba (1947), and that result has since been conflrmed with roots both of Vicia Faba and of Phaseolus. 5. THE NATURE OF THE CHANGES IN TRANSVERSE GROWTH Rawitscher (1932) has suggested a way in which changes in transverse growth might cause a torsion, and his suggestion has been very valuable to the writer. A brief statement of this suggestion was given previously (1942). Essentially it is that the dorsal half of the dorsiventral organ is in respect of its transverse growth negatively geotropic, and its ventral half positively geotropic. Consequently when the organ is laid on one flank its dorsal half begins to curve upwards in the transverse direction, and its ventral half to curve downwards, and these responses bring about the torsion. As a help towards grasping this idea, one can imagine a row of little seedlings which stand erect in the dorsiventral plane of the organ when it is normally orientated, and curve up with their shoots and down with their roots when the organ is on its flank. The writer tested this suggestion by arranging various petioles on their flanks and keeping them flxed for several days so that they could not twist more than a little, and then examining their changes of shape in transverse sections. At first (1942, p. 8; 1945) the evidence seemed to support Rawitscher's suggestion, since the dorsal parts were often found to have curved or crept upwards, and this was shown especially clearly by the dorsal ridges of petioles of Phaseolus (1945, Figs. 9, 10). But later (1946) it was found that the cortex of the ventral halves of various petioles could also be seen to have curved or crept upwards when lines marking its original position were available, whereas according to Rawitscher's suggestion they should have curved downwards. So his suggestion can hardly be right in its original form, but one could save a part of his idea by supposing that both halves of the organ tend to curve upwards in the transverse direction when it is on itsflank, but that the dorsal half does so the more strongly. This modified form of his suggestion would have the further advantage that, unlike the original suggestion, it could be applied also to the auxin torsions of radial stems; for it could be supposed that both sides of a stem when horizontal tend to creep upwards in the transverse direction, but that the side treated with auxin does so the more strongly. Indeed, it was shown that in a pea stem flxed horizontal the ridges of the cortex which are in the horizontal plane do tend to creep upwards relatively to the stele (1946, Fig. 11).

10 154 ^- SNOW 6. THE RESPONSES OF PULVINI It was twice shown previously (1942, 1945, p. 73) that the distal pulvini of PhastoLui, leaves, when fixed distally and arranged on their flanks, twist negatively so as to lower the dorsum, and this result has since been conflrmed once more. Also the pulvini of leaflets of Wistaria were found to react in the same way (1942). So the strophic torsions of these pulvini must be brought about differently from those of growing organs, and in a way which agrees with Schwendener & Krabbe's theory of a change in the direction of elongation of cells. Another difference is that the pulvini of Phaseolus do not twist when treated with auxin on one flank but only curve away, as was mentioned above. This indicates that the vigorous strophic torsions of these pulvini, like those of the petioles, cannot result from their curvatures. However, the process of torsion in pulvini may not be in detail exactly what Schwendener & Krabbe suggested, since pulvini respond by changes of turgor, and consequently contractions need to be considered as well as elongations, Pulvini of other families need to be studied also. SUMMARY 1. Two questions are discussed in the light of evidence presented here or previously, concerning the natural geostrophic torsions of growing organs and also those caused by auxin applied to one flank when they are horizontal or inclined. The latter are called 'auxin torsions'. The first question is, what are the internal changes in auxin conditions leading to these torsions? The second is, what is the primary growth change in the movement? 2. One of the results reported here is that petioles of Phaseolus make no auxin torsions on the clinostat. Those made by sunflower petioles are reported and analysed. It is also shown that for an auxin torsion in coleoptiles the effect of the applied auxin needs to be roughly balanced with the strength of the geotropism. 3. It is concluded that a torsion is set going when the dorsum of a dorsiventral organ, or any face of an organ to which auxin is applied externally, is not opposite to another face towards which auxin is being diverted by the transverse stimulus of gravity or presumably of light. It is discussed whether the next stage in the process may be an unbalanced transverse flow of auxin (Figs. 1-3). 4. It is also concluded that the primary growth change in these organs is a change in the distribution (not direction) of transverse growth, and it is discussed what this change may be. But the torsions of the pulvini investigated are brought about differently, and may be due to a change in the direction of elongation. REFERENCES MUNCH, E. (1937). Entstehungsursachen und Wirkung des Druck- und Zugholzes der Baume. Part 2, Silva, 25, 345. MUNCH, E. (1938), Untersuchungen iiber die Harmonie der Baumgestalt. J. zviss. Bot. 86, 581. RAWITSCHER, F. (1932). Der Geotropismus der Pflanzen. Jena. SCHWENDENER, S. & KRABBE, G. (1892). Untersuchungen liber die Orientierungstorsionen der Blatter und Bluten. Abh. Akad. Wiss. Berlin, Math.-nat. KL, Abt. i, i. SKOOG, F. (1937). A deseeded Avena test method, etc. J. Gen. Physiol. 20, 311. SNOW, R. (1942). Torsions and their analysis. Neto Phytol. 41, i. SNOW, R. (1945). Further experiments on torsions of leaves. New Phytol. 44, 170. SNOW, R. (1946). The direction of torsion and the changes of shape in leaves, etc. New Phytol. 45, 25. SNOW, R. (1947). Torsions induced by auxin. Neio Phytol. 46, i. UYLDERT, I. E. (1931). De invloed van groeistof op planten met intercalaire groei. Dissertation, Santpoort. {Accepted 20 February 1949)

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