A TEST OF SACHS'S THEORY OF THE PLAGIOTROPISM OF LAMINAE

Transcription

1 [25] A TEST OF SACHS'S THEORY OF THE PLAGOTROPSM OF LAMNAE BY R. SNOW Fellozv of Magdalen College, Oxford (With 5 figures in the text) According to a suggestion of Sachs (179, pp. 246 and 254) the movements of plagiotropic members of plants in general can be understood if these parts are thought of as built up of a number of narrow transverse orthotropic components standing in and parallel to the plane of dorsiventrality (Fig. i). The morphologically upper ends of these components must be supposed to curve when stimulated like orthotropic stems, and their lower ends like orthotropic roots. The curvatures of the components will then bring about the observed curvature of the whole plagiotropic member (Figs. 2, 3). This suggestion was based on an earlier suggestion made by Stahl (quoted by Sachs, p. 252) with regard to lichens. t could be applied most simply to the movements of laminar organs, since in many lateral stems and petioles the normal orientation depends mainly or entirely on a balance between negative geotropism and epinastism. Sachs indeed referred to his suggestion in one passage (p. 246) as a 'way of considering' (Vorstellungsweise). But whatever its nature may be and whatever the exact details, there is a simple way, which has apparently escaped notice, in which it can be tested by its consequences; and an experiment to test it on leaf blades of Stachys silvatica will be reported here. The question whether Sachs's suggestion is sound is clearly still a live one since, for example, Rawitscher (1932) has tried to explain the torsions starting from a similar idea. The leaves of S. silvatica respond well to light, and in the usual manner. n overhead light the blades of the younger expanded leaves are about horizontal, and their petioles are inclined upwards at angles from 6 to 7. f a plant is placed in a box open on one side, so that it receives the diffuse daylight from that side coming obliquely from above, the leaves curve as follows. n a leaf which is in front that is, towards the open side of the box with its tip towards the light (see Fig. 4), the petiole becomes dorso-convex and so sinks. The main part of the blade, between its tip and the petiole also curves dorsoconvex, and the extreme basal part, the auricles, sometimes curves slightly dorsoconcave, so that the whole blade makes an 'S* curve towards the light. n a leaf which is at the back with its tip away from the light these three parts all curve in the opposite senses and in the leaves which are in the lateral positions the petioles twist and the blades may also make 'S' curves towards the light in the transverse direction. The curvatures of the blades are rather slight, especially the dorso-concave curvatures, but they become stronger if the petioles are fixed. Also in S. silvatica the auricles are short and so do not curve much. Similar curvatures of blades in other species have been well described by Raydt (1925). t is necessary that very little light should strike the lower surfaces of the

2 Test of Sachs's theory of the plagiotropism of laminae 259 leaves, especially of the ' back' leaves, since if more than a little light does so, it causes in all parts alike very strong dorso-convex curvatures which obliterate all the others. These last curvatures deserve further study. The curvatures of the blades in response to light striking their upper faces obliquely can be interpreted on the basis of Sachs's theory. Thus, for example, in the main part of the blade of a leaf which is in front, with its tip towards the light, the transverse components, tending to turn their upper ends towards the light and their lower ends away, will tend to stretch the upper half of the blade and compress the lower half (see Figs. 1-3), and so may be supposed to make this part curve dorso-convex. But in the auricles of this leaf and in the main part of the opposite leaf with its tip away from the light the same Figs. 1-3 illustrate Sachs's theory of plagiotropism. Fig. i shows the transverse components unstimulated; Fig. 2 shows them when stimulated by oblique light; Fig. 3 shows the resulting curvature of the lamina. The arrows show the direction of the light. Fig. 4 shows a pair of leaves of Stachys silvatica in oblique light. The serrations of their edges are omitted. curvatures of the transverse components will compress the upper half and stretch the lower half, since these parts are attached at their ends nearer to the light; and so the curvatures of the components will tend to make these parts curve dorso-concave. Thus on Sachs's theory the sense in which any part of a blade curves must depend on the end at which it is attached. n order to find out whether this is so, the following experiment was performed (see Fig. 5). From the median parts of the blades of young recently expanded leaves of plants of S. sitvatica, when the blades had just descended to the horizontal position, flaps were cut out by two cuts parallel to the mid-rib and a third transverse cut through the mid-rib near its basal end. At their apical ends the flaps remained attached to the rest of the blade. They were mostly from 3 to 4 cm. long and from 2 to 3 cm. wide and each was supported on a thread near its free end. The petioles were fixed in position at their distal ends, and

3 26o R. SNOW the plants, which had been growing in overhead light in a greenhouse, -were placed in boxes lined with black paper and open on one side towards the north, so that the leaves operated upon were either in front with their tips towards the light, or behind with their tips away from it. Through this operation the parts of the blades forming the flaps were now attached only at their far ends, and consequently on Sachs's theory they should curve in response to the light in the sense opposite to the normal, if at all. The actual changes of curvature are shown in the table. Some of the leaves, nos. 4-6 of the front leaves and nos. 4 and 5 of the back leaves, had all their apical parts beyond the flaps lightly held between two microscope slides clamped at their far ends, as is shown in Fig. 5. The purpose of this arrangement was to provide a firmer attachment against which the strips Fig. s. A leaf of Stachys silvatica with a distally attached flap cut out from it and supported on a thread, and with the apical part of the leaf held between glass slides. might react: but actually it was not found to alter the result, and so was soon given up. Those leaves which were not held between slides were supported on rough black paper instead, except front leaf no. i, which was supported on threads. The duration of the experiment was about 31 hr. for most of the leaves, but from 4 to 54 hr. for front leaves nos. i and 4, and back leaf no. 3. After 31 hr. the curvatures usually did not change any more. The curvatures of the flaps were taken to be the curvatures of their mid-ribs. They were measured well before the evening sleep movement began. The experiments were made on only one leaf of each plant at a time, so that this leaf might always be at about the same distance from the open side of the box. Table i shows that in the front leaves the flaps all curved dorso-convex, just as do the corresponding parts of intact leaves, though on Sachs's theory they should have curved dorso-concave if at all. Also all except one of the Haps of the back leaves, which on that theory should have curved dorso-convex, curved either dorso-concave or scarcely at all. t need not cause surprise that they often failed to curve at all, since even in intact

4 Test of Sachs's theory of the plagiotropism of laminae 261 leaves the dorso-concave curvatures are often feeble. A few similar experiments with leaves of Litnaria biennis gave similar results. Sachs's theory therefore does not hold for the phototropic curvatures of these leaf blades, and probably not for the very similar curvatures of those of other species either, though possibly it might be found to hold for such different objects as the thallus lobes of liverworts with which he experimented. t therefore needs to be considered what empirical rule would indicate correctly the senses of the phototropic curvatures of leaf blades in different conditions. f the region of attachment of a blade to its petiole may be called its centre, though not geometrically so, the rule appears to be that light striking Table i. Stachys silvatica. Changes of curvature in degrees of distally attached flaps of leaf blades in oblique light from above. Changes of curvature towards increased dorso-concavity or decreased dorso-convexity are recorded as positive Leaf no. Front leaves Curvature at start Change of curvature Leaf no. Back leaves Curvature at start Change of curvature Dorso-convex 25 % Dorso-convex 1 <, 12 ", Dorso-concave Dorso-convex 1 % 5 2 )» 2 Dorso-concave 13 i> 1 Dorso-convex 7 1 -l-io Mean Mean + 6 obliquely the upper face of a blade tends to make the parts of it which it strikes centripetally to curve dorso-convex, and those which it strikes centrifugally to curve dorsoconcave. This rule is further supported by the observation that in Stachys silvatica and other species the curvatures, both convex and concave, induced in the leaf blades by oblique light from above are commonly made not only in the direction of the light, but also to some degree in the transverse direction. On Sachs's theory this would not be expected. SUMMARY 1. From Sachs's theory that the plagiotropism of a lamina may be regarded as depending on the curvatures of orthotropic transverse components within it, it would follow that the sense in which any part of a lamina curves, whether dorso-convex or dorsoconcave, must depend on whether that part is attached by the end towards the stimulus or away from it. 2. t is shown by a phototropic experiment that in leaf blades of Stachys silvatica this is not so, and an empirical rule for the senses of the curvatures of different parts of such blades is proposed as an alternative. REFERENCES RAWTSCHER, F. (1932). Der Geotropismus der Pflanzen. Jena. RAYDT, G. (1925). Ueber die Bewegungen euphotometrischer Blatter. Jb. wiss. Bot. 64, 731. SACHS, J. {179). Ueber orthotrope und plagiotrope Pflanzentheile. Arb. Bot. nst. Wurzburg, 2, 226.

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