JUNE 1947 VOL. 46, No. i TORSIONS INDUCED BY AUXIN BY R. SNOW Fellow of Magdalen College, Oxford (With I figure in the text) It was reported in a previous paper of this series (1945, p. 77) that petioles of various species and plagiotropic stems of Philadelphus, if given a streak of hetero-auxin in lanoline applied on one side, twist so as to raise the treated side up to or towards the top. It was also shown by an experiment on petioles of Phaseolus multiflorus that the rule is that the torsion raises the treated side towards the top: for it did so even when the whole plant was inverted, although the direction of torsion was then the opposite in relation to the structure of the petiole. It was concluded that the treated side comes to act physiologically in relation to gravity like a dorsum; and it was further thought probable that the normal geostrophism of such lateral members, which brings the morphological dorsum to the top, is related to a tendency to accumulate the natural auxin in higher concentration along the dorsum. Fig. I. Phaseolus looked at horizontally after 54 hr. The dotted line was on top at the start, and the paste was on the far side. These results suggested the question whether orthotropic stems, roots and coleoptiles would react in a similar way if laid horizontal and treated with hetero-auxin in lanoline on one side; and accordingly the following experiments were carried out. Four seedlings of Phaseolus multiflorus were grown in pots, and when the epicotyls were elongating rapidly, the paired first leaves being 5 or 6 cm. long, the pots were laid on their sides in overhead light so that the epicotyls were horizontal with the plane of the leaves also horizontal. Then a thin streak of lanoline containing hetero-auxin at a concentration of i in 275 was placed along one side of the distal i-2 cm. of the epicotyl and was continued for a little way along the ventral face of the petiole of the leaf on that side. The four epicotyls all twisted in such a direction so as to raise the paste from the lateral position towards the New Phytol. 46, i '
2 R. SNOW top, the torsions being after 6.^ hr. roughly 75, 6o^\ 30 and 30, and after 23 hr. 80, 60, 60" and 50". The figure shows the third of these seedlings drawn as seen horizontally after 54 hr., when it had twisted 80. Four seedlings of Pisum sativum were similarly placed horizontal, with the plane of the leaves horizontal, and in each of them a young internode, only about 2 cm. long, was given a thin streak of the same paste along its distal half on one side. The paste was continued apically for a little way along the petiole on that side. In all four seedlings the treated internodes twisted so as to raise the paste, the torsions being after 6J hr. 90", 80, 80 and 70, and after 23 hr. 90", 90^ 90 and 80. Two young sunflower seedlings, with hypocotyls 5 and 2-5 cm. long, were placed horizontal with the plane of the cotyledons horizontal, and a very thin streak of a heteroauxin paste of i in 400 was placed along the distal i cm. of each hypocotyl on one side and for a little way along the petiole of the cotyledon on that side. The hypocotyls twisted so as to raise the paste, the torsions being 90 and 70 after 23 hr. Three young tomato seedlings, with hypocotyls only 2 cm. long, were treated like the sunflower seedlings, but with the paste of i in 275. The hypocotyls all twisted so as to raise the paste, the torsions being after 30 hr. 50", 35 and 15", and after 54 hr. 50, 50 and 25. Six oat seedlings grown in the dark were placed so that the elongating coleoptiles were horizontal with their longer transverse diameters horizontal, and each was given a streak of this I in 400 paste along one side. V\\e coleoptiles all twisted so as to raise the paste, the torsions being after 23 hr. at about i7'5 C, iio^, 90", 90, 90'^, 90" and 90. Eight young seedlings of Vicia faba, with main roots still quite short, were each given a streak of a hetero-auxin paste of concentration only i in 6000 on one of the narrow sides of the root. The paste reached from the level of the apical meristem to about 1; mm. behind it. The seedlings were then replaced in loose damp sawdust so that the roots were horizontal with their two narrow sides horizontal. Little shields prevented the sawdust from falling on to the treated apical parts of the roots. The roots all twisted so as to raise the paste, the torsions being after 23 hr. 70, 65, 50, 45, 40, 40, 30 and 25. Thus all the organs tested twisted so as to raise the sides treated with hetero-auxin paste. Naturally the various organs made curves as well as torsions, the curves of the stems and coleoptiles being at first curves away from the paste in the horizontal plane and geonegative curves in the vertical plane. In the four Phaseolus epicotyls, for example, at 6-J hr. the horizontal curves vi'ere 60""', 50", 40^ and 20, and the vertical curves 45, 25, 20 and 30^. But by this time they had made torsions raising the paste which ranged from 30 to 75, so that from this time onwards the tendency to curve away from the paste was directed obliquely downwards and partially counteracted the negative geotropism. Consequently after this time the curves in the two planes did not increase much more, and some of them diminished. Similar results were noticed in the other species also. The various orthotropic stems and coleoptiles which were placed horizontal and had one side treated with hetero-auxin paste were thereby made to curve as well as twist like plagiotropic stems; and this resemblance was not only in the final result, but in the process leading up to it. For it is now generally accepted that normal plagiotropic stems, and also leaves, commonly owe their orientation to a balance between negative geotropism and an opposing tendency which may be called epinastism. Correspondingly in the stems
Torsions induced by auxin 3 and coleoptiles treated with auxin paste, the curvature away from the treated side, which became physiologically dorsal, opposed the geonegative curvature after the treated side had risen by torsion, just as does in natural plagiotropic stems the epinastic curvature away from the natural dorsum. But it remains to be discovered how it comes about that the face of a stem or petiole which is physiologically dorsal, either naturally or through being treated with hetero-auxin, tends to rise by torsion to the top. The fact that the roots, though positively geotropic, twisted like the stems so as to raise their treated sides need not be surprising, since in roots as in stems auxin is diverted towards the lower side. In the roots the horizontal curvatures were rather strong, and were of course towards the paste. Incidentally it was noticed that in the stems of Phaseolus and Pisiim the torsions and the curves away from the paste were carried out by the auxinated zone of the stem itself and by a rather short zone just basal to it, whereas the geonegative curves were carried out by a zone further towards the base which was mainly or entirely separate. Consequently, after a day or more, when the stems had twisted so that the two curves were nearly in the same plane, the total curve was S-shaped, as is shown in the figure; and similar S curves were noticed in the coleoptiles. It needs finally to be considered whether the torsions may have been caused by the two curvatures. This is made unlikely by the fact that in Phaseolus and Pisitm the two curves, when present, were mainly in different zones. Moreover in two of the Pisiim stems the geonegative curves at 6?. hr. were nil, and in the other two only 25 and 35'. But it seemed desirable to test directly whether in similar stems curvatures in two planes do cause torsions. So seven similar Phaseolus seedlings were arranged with their epicotyls horizontal, and when these had curved up about 60 the pots were rotated through 90, so as to stimulate them in the plane at right angles. When the components of the total curvatures in the new vertical plane were from 40 to 75 (mean 47 ), it was found that the earlier curves, which were now the components in the horizontal plane, were still from 20 to 35 (mean 29 ); but there were no torsions. With the same method two short young sunflower hypocotyls were stimulated in two planes at right angles, the component curves being in the one hypocotyl 45 and 80 simultaneously, and in the other 20 and 45 ; but again there were no torsions. So in young stems of Phaseolus and Helianthus similar to those used in the experiments with auxin paste, two curvatures induced in planes at right angles are not necessarily accompanied by a torsion, and it therefore seems that in the experiments with the paste the torsions cannot have been caused by the two curvatures. The results of these last experiments are contrary to the statement of Staub (1934) that in many organs, including hypocotyls of Helianthus, a convex face acts as if physiologically dorsal. For if so the face which became convex in the earlier geotropic curvature should have risen by torsion when the pots were rotated through 90. But it seemed possible that longer and weaker sunflower hypocotyls, such as would sag when placed horizontal, might twist when similarly stimulated in two planes. For Rawitscher (1932, p. i()'() mentions that many long shoots, which are too weak to raise their own weight by geotropism when horizontal, make torsions. Actually it was found that two longer and weaker sunflower hypocotyls when similarly stimulated in two planes did make torsions of 30 and 70 such as to raise the convex side of the earlier curve. Whether any of the torsions
4 R. SNOW reported by Staub can have been caused in this way is not clear; but the torsions reported here, which were induced by hetero-auxin paste in organs that were not sagging, cannot anyhow have been so caused. SlIMMARY 1. Various young orthotropic stems, and also coleoptiles and main roots, when placed horizontal and treated on one side with hetero-auxin paste all twist so as to raise the treated side to the top, or towards it. 2. These torsions are not caused by the curvatures in two planes which are also induced. 3. The organs treated with the paste become plagiotropic, the treated side being physiologically dorsal. REFERENCES RAWITSCHER, F. (1932). Der Geotrnpismus tier Pflan::en. Jena. SNOW, R. (1945). Further experiments on torsions of lea\'es. Neio Phvtol. 44, 70. STAUB, H. (1934). Beitrag zur Kenntnis unnnittelbarer Torsionen, etc. Schu-eiz. Bot. Ces. 43, 191.