PHYLLOTAXIS AND APICAL GROWTH

Transcription

1 PHYLLOTAXIS AND APICAL GROWTH BY ELIZABETH G. CUTTER Department of Botany, University of Manchester {Received 12 May 1963) SUMMARY Apical size, phyllotaxis index and the rate of leaf inception v^^ere compared in spiral and bijugate specimens of Dryopteris from the same collection and were found to be similar. Apices were laid bare and maintained under observation in the laboratory for periods of up to 9 months; during this time the bijugate specimens became spiral, as previously reported. Exploratory experiments on the bijugate genus Dipsacus indicate that treatment with gibberellic acid leads to increased internodal elongation and a slight increase in the mean angle of divergence. Removal of young developing leaves and leaf primordia leads to an increase in the rates of leaf inception and development. These observations are discussed in relation to the earlier observations and experiments of Snow^. The conclusion is reached that while apices with different systems of phyllotaxis may be of similar size and have similar rates of leaf formation and radial growth, progressive changes in various rates of growth may nevertheless lead to changes in phyllotaxis by modifying the relationships between successive leaf primordia. Vertical as well as radial and tangential growth should be taken into account. INTRODUCTION One of the most conspicuous features of apical growth in the shoot is the formation of leaf primordia in a regular and characteristic sequence or pattern. The occurrence of more than one such pattern, or phyllotactic system, within a species is not uncommon. To cite a recent example. Tucker (1962) has described the occurrence in Michelia fiiscata of both spirally organized leading shoots and dorsiventral branch shoots with leaves in two rows. Other examples are described in the earlier literature. Deschatres (1954) reported the occurrence of both bijugate and Fibonacci spiral phyllotaxis in Sedum elegans. Loiseau and Deschatres (1961) described stems of S. elegans and S. reflexum which show a gradual transition from spiral phyllotaxis of the accessory series I, 3, 4, 7, II,... to a bijugate system. In Michelia, also, vigorous branch shoots with dorsiventral symmetry change spontaneously to radially symmetrical shoots with spiral phyllotaxis (Tucker, 1962). In the fern Dryopteris dilatata (Hoffm.) A. Gray {D. aristata (Vill.) Druce) a proportion of specimens exhibit bijugate phyllotaxis and not the more typical Fibonacci spiral arrangement (Voeller and Cutter, 1959). Since this report, a bijugate specimen of D. filix-mas has also been noted and three naturally occurring specimens of D. dilatata with spiral phyllotaxis from the accessory series i, 3, 4, 7, 11,... have been observed. The occurrence of more than one system of phyllotaxis within a species affords an opportunity for comparative growth studies in relation to phyllotaxis, and this paper is mainly concerned with such a study in Dryopteris. In an important survey of phyllotaxis, Richards (1948, 1951) analysed the radial spacing of primordia in relation to processes of growth occurring in the apex. By means of the phyllotaxis index, which is a simplified mathematical term derived from the plastochroife 39

2 40 ELIZABETH G. CUTTER ratio (the ratio of the distance of one primordium from the centre of the apex to that of a primordium one plastochrone younger), different systems of phyllotaxis, mcluding ^^ h<n-led and spiral systems, can be directly compared. Successive integers of the phyllotaxis index represent the various orthogonal systems of Church. Specimens of Dryopteris with bijugate phyllotaxis which have had all the expanded and the majority of the unexpanded leaves removed, and which have been mamtained under observation, ultimately change to a Fibonacci spiral arrangement, or, less frequently, to other systems (Cutter and Voeller, 1959). These changes, which are accompanied by a fall in phyllotaxis index, are sometimes preceded by an increase in the angle of divergence between successive pairs of primordia, a result comparable to that obtained by Snow (1951) in experiments on the bijugate species Dipsacus laciniatus. In this investigation Snow removed the young developing leaves and leaf primordia, sometimes down to the youngest pair, leaving some mature leaves intact, in order to test Richards's (1948) suggestion that in bijugy the deviation of a pair of primordia from a position at right angles to that of the preceding pair is due to inhibition from the second older pair. These experiments, like those on Dryopteris, thus involved excision of the young, developing leaves and leaf primordia. In the present investigation, some of the effects of leaf excision on growth in Dipsacus were briefly examined, and the effect on phyllotaxis of an increase in the relative amount of elongation, brought about in intact plants by treatment with gibberellic acid, is described. MATERIALS AND METHODS Bijugate and spiral apices of Dryopteris dilatata from the same collection were laid bare by removing all but about twelve of the youngest leaf primordia, and maintained in pans of peat under constant conditions of temperature (about 23 C). They were then observed under a stereoscopic microscope at approximately fortnightly intervals for several months, and a record kept of the new primordia formed. The plastochrone ratios were measured from camera lucida drawings, and the phyllotaxis index calculated (Richards, 1951). Where the mean values were derived from ten specimens or less, the 95% confidence interval was used instead of the standard error. Some experiments were also carried out on young plants of Dipsacus spp. grown in pots. In some experiments plants were sprayed with an aqueous solution of gibberellic acid (GA) at 10 ppm, control plants being sprayed with water. Apices of Dipsacus were finally embedded and sectioned, and measurements of angular divergences, etc., made from drawings of the sections. Pairs of visible leaf primordia are called Pi's, P2's, Ps's, etc., the Pi's being the youngest; those which were invisible at the beginning of the experiment but appeared during its course are called Ii's, 12's, etc., the Ii's being the first to appear (Snow and Snow, 1931). RESULTS Observations on Dryopteris Under the conditions of the experiment there was no difference in the rate of leaf inception in spiral and bijugate apices of Dryopteris (Fig. i). For the sake of clarity, the confidence intervals have been omitted from the graph; however, the two curves were not significantly different at any point. During the first 130 days the mean plastochrone in the spiral apices was 7.81 ± 1.16 days, and in the bijugate apices 7.62 ± 1.13 days. By this time some of the initially bijugate apices had in fact become spiral; if the plasto-

3 Phyllotaxis and apical growth 41 chrone is calculated only from those specimens still bijugate at this time the value becomes 7.74 ± 2.34 days. In both spiral and bijugate apices, the rate of leaf inception was fairly uniform, but declined slightly during the period of observation. In the second period of 90 days the mean plastochrone in both groups was a little over 3 days longer than in the first such period. (Spiral apices: first period, 7.99 ± 2.27 days; second period, ±2.19 days. Bijugate apices: first period, 7.35 ± i-i3 days; second period, 9.48 ± 1.45 days.) If the rate of leaf inception is plotted for individual bijugate specimens which became spiral, or which gave rise to trimerous pseudo-whorls, or spiral systems from the accessory series i, 3, 4, 7, 11,... (Cutter and Voeller, 1959), the change in phyllotaxis does not appear to have been accompanied by any marked disruption of the approximately uniform rate of formation of new leaf primordia c c ni Days Fig. I. The rate of leaf inception in spiral (O) and bijugate ( x) apices of Dryopteris dilatata maintained under the conditions described in the text. The phyllotaxis indices of spiral and bijugate apices from the same collection were also compared. The mean phyllotaxis index of ten spiral apices at the time of collection was 3.70 dz o.io; that often bijugate apices was 3.73 ± It may be concluded that there is no significant difference in phyllotaxis index, i.e. in the radial spacing of the primordia, in bijugate and spiral apices of Dryopteris. There is also apparently no difference in the mean angle of the apical cone (average for spiral apices, 53 ; range : average for bijugate apices, 55 ; range, ) but these measurements were obtained simply by drawing the living apices in profile, and are likely to be rather inaccurate. If there is indeed no difference in the angle of the apical cone, and in phyllotaxis index in the two groups of apices, this would mean that there is no regular difference in equivalent phyllotaxis index (E.P.I.), a measure of the curve systems on the apical cone itself (Richards, 1951). This value was not calculated for the individual specimens, however, and may vary considerably. Since the apices were kept under observation and not sectioned, the method of calculating E.P.I, described by Richards (1956) could not be employed.

4 42 ELIZABETH G. CUTTER During the course of the experiment the phyllotaxis index of both spiral and bijugate apices decreased (Fig. 2). This decrease took place in a parallel fashion in the two groups of apices. By plotting the phyllotaxis index of individual specimens which underwent changes in phyllotaxis it can also be shown that these changes were not reflected in any sudden change in the curve. When factors affecting phyllotaxis are constant, the radial relative growth-rate ot the apex in the region of leaf formation is equal to the natural logarithm of the plastochrone ratio divided by the plastochrone period (Richards, 1951). Since these parameters have been shown to be comparable in the two groups of specimens, it may be concluded that at a particular moment, e.g. at the time of collection, the radial relative growth-rate of bijugate and spiral apices of Dryopteris is similar. Moreover, Figs, i and 2 together indicate that the radial relative growth-rate of bijugate and spiral specimens must have undergone comparable changes over a period of time Weeks Fig. 2. The change in phyllotaxis index with time in spiral (O) and bijugate ( : ) apices of Dryopteris dilatata maintained under the conditions described in the text. The arrow marks the time by which all the bijugate apices had become spiral; the vertical bars represent the fiducial limits at the P=o-O5 level. When the size of the bare apex was calculated according to the regression method of Richards (1951), there was no apparent difference in apical size in bijugate and spiral specimens at the time of collection. Observations on Dipsacus In general, in Dipsacus the excision of young, growing leaves and leaf primordia led to an increased rate of leaf formation during the period of the experiment; excision of expanded leaves only had no effect. Various treatments were carried out, involving, for example, the immediate excision of all visible leaf primordia, or excision down to the P4's, followed by continued excision of primordia as they reached this stage (expanded leaves were left on the plants). Fig. 3 indicates that in D. strigosus and D. laciniatus there was an approximately proportional relationship between the total number of pairs of leaves excised and the number of pairs of new primordia formed during the period of the experiment, in this case 7 weeks. Defoliation thus stimulated leaf inception and development, under these conditions. The phyllotaxis of all the plants, under any treatment, remained bijugate during the period of the experiment.

5 Phyllotaxis and apical growth 43 Intact plants sprayed with gibberellic acid (GA) showed some extension of internodes, which in rosette plants of Dipsacus are normally very short. In plants of D. fullonum, given two treatments with a 7 day interval and then allowed to grow on for a further 5 weeks, some extension took place of the internodes between Ii's and Pi's, Pi's and P2's, etc., for several internodes. However, the youngest internodes, formed during the course of the experiment, were not appreciably affected. In D. sylvestris, also, the youngest internodes present at the end of the experiment had not perceptibly elongated, but slightly older ones showed some extension. The effect of GA treatment on the mean angle of divergence is shown in Table i. 112 ro I 8 Q. xo 0 o A 8 12 No. ot pairs of leaves removed Fig. 3. The effect of partial defoliation on the number of pairs of new leaf primordia formed in 47 days in Dipsacus strigosus (O) and D. laciniatus ( x). Each point represents one plant. Table i. The effect of treatment with gibberellic acid on the mean angle of divergence in Dipsacus Species Treatment Mean angle ±S.E. D. fullonum Control 73.2 ±1.79 GA: two treatments with 7-day interval 76.1 ± D. sylvestris Control GA: seven treatments with approx. 5-day intervals 71.6 ± ±0.79 DISCUSSION The occurrence of a proportion of specimens with bijugate phyllotaxis in a normally spiral species suggested that a study of phyllotaxis in relation to apical growth might yield a possible explanation of the factors controlling bijugy, and also of the change from bijugy to other systems of phyllotaxis which occurs under the conditions described. It appears, however, that differences in leaf arrangement in bijugate and spiral specimens of Dryopteris cannot be attributed to, or correlated with, differences in (i) the rate of leaf formation; (ii) the radial spacing of the primordia; (iii) the rate of radial growth of the shoot apex; or (iv) the size of the apical meristem. Indeed, as Richards (i951) has already demonstrated theoretically, it appears that bijugate specimens are simply double versions of Fibonacci spiral specimens, differing essentially only in the approximately synchronous formation of two leaf primordia at each node.

6 44 ELIZABETH G. CUTTER These observations on Dryopteris seem worth recording, however, since few data of this kind are available. Schuepp (1916) reported that the plastochrone in decussate specimens oihehanthus anmtus, measured between successive pairs, was 11 days, whereas it was 5.6 days in spiral specimens. Priestley and Scott (1933) showed that the plastochrone for a pair or a whorl of three leaves in H. tuherosus was about 5 days, whereas in spiral specimens it was about 2 days. In both these instances the average time taken for the formation of one leaf primordium in specimens with differing p>hyllotaxis is approximately equal, which is in agreement with the results now reported in Dryopteris. Tucker (1962) also states that shoots of MicheUa with different phyllotaxis produce leaves at comparable rates. Richards (1951) has examined the results obtained by Snow and Snow (1935) after bisection of decussate apices of Epilobium hirsutwn, and has shown that in one specimen, in which the two halves regenerated apices with spiral and decussate phyllotaxis respectively, there was no difference in rate of leaf inception or in phyllotaxis index in the two shoots produced. He concluded that the change from a decussate to a spiral system of phyllotaxis had occurred without any noticeable change in basic processes of growth, and that the 'actual pattern on apices of the same size is to a large extent incidental'. The observations on Dryopteris described above serve to substantiate these conclusions. Some explanation must, however, be sought for the curious fact that, whereas spiral apices of Dryopteris maintained under observation in the laboratory for periods of up to a year merely change to lower spiral systems (Cutter, 1955), similarly treated bijugate specimens undergo a change in phyllotaxis in a shorter time, usually to Fibonacci spiral systems. Bijugy is evidently unstable in Dryopteris under these conditions. While the various parameters of apical growth studied have been shown to be similar in spiral and bijugate specimens, it remains possible that progressive changes in these parameters may have a more evident effect on one system of phyllotaxis than on the other. The observed change from bijugy to spiral phyllotaxis is usually a result of a pair of leaf primordia becoming unequal in size and situated at slightly different levels. Camefort (1956) has described a bud of Seqitoia sempei'virens in which a transition from bijugy to a Fibonacci spiral arrangement took place in a similar fashion. In Dryopteris, bijugy is probably only approximate; and existing slight differences in size and level of insertion between the two primordia of a pair could be enhanced by a lengthening of the plastochrone, which has been shown to occur over a period of time, and probably also by increased internodal grovi1:h, which seems also to occur under the experimental conditions. It has previously been shown that in spiral apices of Dryopteris maintained under the conditions described over a considerable period of time, the differences in size and stage of development between successive primordia are increased (Cutter, 1955). A similar effect on existing small differences between the two primordia in a pair of bijugate apices might account for the change to a spiral system. On this view, the change in phyllotaxis would be a result of changes in the rates of the various correlated components of growth in the apex occurring over a period of time. It was not due to treatment of primordia adjoining the sites of new primordia, nor did the change in phyllotaxis occur for some time after the experimental treatment (namely, laying bare the apex). It is known that phyllotaxis can be affected to a greater or lesser extent by various external factors, e.g. photoperiod, vernalization, etc., which may also affect rates of growth. In Dipsacus, bijugy is evidently much more stable than in Dryopteris since changes to spiral or other systems of phyllotaxis were rarely observed by Snow (1951) in a consider-

7 Phyllotaxis and apical growth 4r able series of experiments, and never by the writer in the present series However Snow (1951) showed that specimens of Dipsacus defoliated down to one or a few pairs of primordia did show an increase in divergence angle that was more than transitory He interpreted his results in terms of contact between leaf primordia, or their united rims, and the pressure exerted by the Pa's on the shoot apex, factors which cannot be operative in Dryopteris. Snow considered, but rejected, the hypothesis that the changes m phyllotaxis in Dipsacus might be due to changes in the conditions of growth. However, in the present experiments it has been shown that defoliation can result in an increased rate of leaf inception, and other growth changes may also have been involved. Snow (1951) himself found that in intact plants of Dipsacus which had been grown in an inverted position for 16 days, during which 'the apical buds were much retarded in growth', there was a decrease of the angles of divergence below the normal value. The experiments in which intact plants of Dipsacus spp. were treated with gibberellic acid were carried out in order to discover whether increased internodal elongation could affect phyllotaxis in this genus. Rather few plants were used, and angles of divergence in both control and treated plants were variable; it is also perhaps unfortunate that different species from that used by Snow were employed. The increase in divergence angle in GA-treated plants of D. fullonum is not significant (Table i), but in D. sylvestris with a greater number of treatments the results are significant although not striking. However, in this experiment the GA treatment depressed the rate of leaf inception and this may also have had some effect, x^lthough the evidence from these experiments is certainly not strong, it does suggest that changes in internodal elongation can affect the angle of divergence in Dipsacus. In this instance, this has been achieved without defoliation or other wounding of the plant. The view that vertical growth may be a factor in controlling phyllotaxis is supported by Snow's (1954) observation that there was an increase in the angle of divergence in Dipsacus just before flowering, when the stem began to elongate, and a subsequent decrease in the angle between the bracts of the inflorescence, when there was hardly any elongation. However, Snow's (1951) conclusion that pressure of the P2's on the shoot apex is a factor in controlling bijugy in Dipsacus need not be invalidated, since an increase in internodal extension could reduce or remove this pressure, as indeed Snow (1954) has claimed. In this connection, it may be noted that bijugy appears to occur only when pairs of leaf primordia are formed, and remain, in close proximity, and is maintained only under these conditions. Bijugy normally occurs in rosette plants (Church, 1904; Snow, 1951) and in closely packed inflorescences. The frequent correlation between bijugy and the rosette habit has been well demonstrated by Benoist (1932, 1933), who described a number of bijugate species belonging to the predominantly decussate families Caryophyllaceae and Valerianaceae. Benoist was impressed by the fact that the bijugate species which he observed were all mountain plants with a cushion or rosette habit and very short internodes, and he suggested that their bijugate phyllotaxis might be a modification due to the influence of the mountain climate and a consequence of extreme shortening of the stem. In this connection, it is also of interest that Rochon (1956; cited by Loiseau and Deschatres, 1961) has observed the progressive transition from decussate phyllotaxis at the base of the stolons of Ajuga reptans to bijugy in the rosette, and a return to a decussate arrangement in the inflorescence. The observations described in this paper point to the rather paradoxical conclusion that while apices with difl'erent systems of phyllotaxis may have similar rates of growth, progressive changes in the rates of growth of the kind indicated here although perhaps

8 46 ELIZABETH G. CUTTER not only in those may nevertheless lead to temporary or permanent changes in the phyllotaxis. These changes may be more or less conspicuous, depending on the mitial system. Further studies of phyllotaxis in relation to apical growth are desirable, and it may be suggested that the vertical component of growth should not be neglected. Treatment with gibberellin offers a possible means of investigation which might well be exploited further. REFERENCES BENOIST, R. (1932). La phyllotaxie du Phyllactis rigida Pers. Bull. Soc. bot. Fr., 79, 490. BENOIST, R. (1933). La phyllotaxie chez'quelques especes de Caryophyllacees et de Valenanacees. Bull. Soc. hot. Fr., 80, 367, 563. CAMEFORT, H. (1956). Etude de la structure du point vegetatif et des variations phyllotaxiques chez quelques Gymnospermes. Ann. Sci. nat., Bot., 17, i. CHURCH, A. H. "(1904). On the Relation of Phyllotaxis to Mechanical Laws. Williams & Norgate, London. CUTTER, E. G. (1955). Experimental and analytical studies of pteridophytes. XXIX. The effect of progressive starvation on the growth and organization of the shoot apex of Dryopteris aristata Druce. ^Hw. i?o^. Lone/., N.S., 19, 485. CUTTER, E. G. & VOELLER, B. R. (1959). Changes in leaf arrangement in individual fern apices. J. Linn. Soc. Lond., 56, 225. DESCHATRES, R. (1954). Recherches sur la phyllotaxie du genre Seduvi. Rev. gen. Bot., 61, 501. LoiSEAU, J.-E. & DESCHATRES, R. (1961). Les phyllotaxies bijuguees. Bull. Soc. bot. Fr., 108, 105. PRIESTLEY, J. H. & SCOTT, L. L (1933). Phyllotaxis in the dicotyledon from the standpoint of developmental anatomy. Biol. Rev., 8, 241. RICHARDS, F. J. (1948). The geometry of phyllotaxis and its origin. Syinp. Soc. exp. Biol., 2, 217. RICHARDS, F. J. (1951). Phyllotaxis: its quantitative expression and relation to growth in the apex. Phil. Trans.,B, 235, sog. RICHARDS, E. J. (1956). Spatial and temporal correlations involved in leaf pattern production at the apex. The Growth of Leaves (Ed. by F. L. Milthorpe), pp Butterworth. London. SCHOEPP, O. (1916). Untersuchungen uber Wachstum und Formwechsel von Vegetationspunkten. ^aari. zviss. Bot., 57, 17. SNOW, M. & SNOW, R. (1931). Experiments on phyllotaxis. I. The effect of isolating a primordium. Phil. Trans., B, 221, i. SNOW, M. & SNOW, R. (1935). Experiments on phyllotaxis. III. Diagonal splits through decussate apices. Phil. Trans., B, 225, 63. SNOW, R. (1951). Experiments on bijugate apices. Phil. Trans., B, 235, 291. SNOW, R. (1954). Phyllotaxis of flowering teasels. New PhytoL, 53, 99. TUCKER, S. C. (1962). Ontogeny and phyllotaxis of the terminal vegetative shoots of Michelia fuscata. Amer.J. Bot., 49, 722. VOELLER, B. R. & CUTTER, E. G. (1959). Experimental and analytical studies of pteridophytes. XXXVIII. Some observations on spiral and bijugate phyllotaxis in Drvopteris aristata Druce. Ann. Bot., Lond., N.s.,23, 391.

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