DEVELOPMENT OF THE INFLORESCENCE AND SPIKELETS OF ANTHOXANTHUM ODORATUM L.

Transcription

1 DEVELOPMENT OF THE INFLORESCENCE AND SPIKELETS OF ANTHOXANTHUM ODORATUM L. BY B. C. SHARMAN Department of Botany, Bedford College, Regent's Park, London, N.W.i {Received 4 January 1959) (With Plates 3 and 4 and 2 figures in the text) SUMMARY (1) The sequence of events in the development of the inflorescence and spikelets of Anthoxanthum odoratum is described and illustrated. (2) The shoot-apex becomes longer and bears more leaf primordia during spring, becoming extremely elongated just prior to inflorescence initiation. (3) Primary panicle buds are borne in two rows in the axils of alternate small collarlike leaf primordia. (4) These buds bear rudimentary leaf primordia in whose axils secondary buds are initiated. Similar repeated branching leads to the development of the panicle type of inflorescence. (5) Spikelet differentiation begins at the tip of the inflorescence and later occurs at the ends of the individual panicle branches. (6) The two glumes and the four lemmas arise in an orderly alternate sequence, followed by the two stamen primordia. (7) The stamen primordia appear to be axial in nature and are not leaf primordia. (8) The two stamens are probably the axillary buds of the last two lemmas. (9) The carpel arises on the side of the spikelet apex in the plane of the lemmas and glumes, continuing their strictly alternate sequence. (10) In origin the carpel appears to be homologous with a single leaf. (11) The two stigmas arise one on each side of the carpel as the latter encloses the spikelet tip. (12) In the terminal spikelet the carpel appears to be the last leaf on the whole shoot and is in strict distichous continuity with (a) the lemmas and glumes of that spikelet, (b) the collar-like rudimentary leaf primordia subtending the primary spikelet-branches below, and (c) the vegetative leaves at the base. INTRODUCTION During the course of studies on the developmental morphology of the shoots of various herbage grasses and cereals, dissections of Anthoxanthum odoratum L. (sweet vernal grass) have proved especially interesting because of the ease with which the early stages of the development of the floral parts can be observed. The following account is intended to illustrate the salient features of the development of the inflorescence and spikelets in 60

2 Infiorescence and spikelets of Anthoxanthum odoratum this particular grass, with special attention to the stamens and carpels since, in this species at least, they seem to be of very different origin from what is generally supposed. 6i MATERIALS AND METHODS Shoots were collected at various times throughout the year (and over a number of years), dissected under a binocular microscope and photographed whilst alive, using precisely the same methods as those described earlier for Agropyron (Sharman, 1947). Although material which has been kept in alcohol and is dead yields rather clearer detail and is easier to photograph than living material, the results appear so artificial that they give a very wrong impression of the real appearance of the apieai regions. For this reason, all the photographs shown in Plates 3 and 4 are of living material only. APEX MORPHOLOGY (a) Vegetative shoot Throughout the late autumn and winter the apex is comparatively short, bearing only two or three primordia before one is reached whieh is sufficiently long to overtop the apex. During the early spring, the apex elongates and bears more small primordia, giving an appearance as shown in Plates 3 a and b.* At the end of the vegetative phase, the apex becomes extremely long and carries many primordia, the majority of which appear as small collars. Plate 3c shows such an apex with about twenty primordia but even more extreme examples with up to thirty primordia are easily found. (b) Inflorescence The change over from a long purely vegetative apex to a young inflorescence, as seen in Plate 3d, is extremely rapid, and so far it has proved impossible to find the presumed intermediate stage with small bud primordia each subtended by its small collar-like leaf primordium (i.e. the so-called 'double ridge' stage) which is so characteristic of infiorescence initiation and so easily found in grasses such as Agropyron, Triticum, etc. Plate 3d shows the distichously arranged primary buds of the future panicle. These bear small ridge-like leaf initials, in whose axils will arise secondary branch primordia, themselves bearing leaf primordia which will subtend tertiary bud primordia and so on. This repeated branching, which may be seen in Plate 3e, causes the infiorescence of Anthoxanthum to be in the form of a panicle, instead of a spike as in, for example, Agropvron or Lolium. The number of original buds which give the clusters of panicle branches, and the degree to which they undergo secondary, tertiary, etc., branching, varies. It is presumably at least partly dependent on the vigour of the particular shoot, but it may also be related to the infiuence of such factors as temperature and day length: shoots developing later in the spring seem to have less original primary panicle buds, and these themselves seem to undergo rather less repeated branching. The process is brought to an end first in the main axis when the tip of the infiorescence differentiates into a single terminal spikelet, as illustrated in Plate 3f. Later similar spikelet differentiation terminates the growth of the tips of the panicle branches. * It is impossible to give any useful dates for the various stages because there is considerable variation between individual shoots even when they are taken from a single clone.

3 62 B. C. SHARMAN THE SPIKELETS (a) Adult morphology Before describing the development of the spikelets in Anthoxanthum, it is perhaps desirable to give a brief outline of their adult morphology, because they have a number of unusual features which themselves help to make this species so interesting. The mature spikelet (shown diagramatically in Fig. ib) bears two glumes at the base, followed by a series of four more chaff members which may be termed lemmas. All six of these parts are arranged in an alternate sequence in two rows (i.e. distichously). Above and in the same plane as these come two stamens (there are no lodicules) and finally a single carpel which bears two stigmas in the plane at right angles to that of the glumes, lemmas and stamens. Fig, I, Diagrams of a, a typical grass spikelet (with four flowers), b, the single flowered spikelet of Anthoxanthum, and c, an alternative interpretation of the s,p\\!ie\et oi Anthoxanthum. For further explanation, see text, C, carpel; G, glume; L, lemma; Lo, lodicules; P, palea; S, stamen. Since there is no difference in appearance or origin between the first three lemmas and the last immediately below the stamens, the term 'lemma' is used to cover all four. From the point of view of analogy, the last member might perhaps be termed the 'palea', but this term is usually associated with the bi-keeled structure which is in reality the first leaf (the prophyll) of the floret borne in the axil of a lemma, as illustrated diagramatically in Fig, I a for a typical grass spikelet. For this reason the term 'lemma' is used for all of the four members found in the spikelet in Anthoxanthum. (b) Developmental morphology Plate 3h shows the tip of a young inflorescence (one intermediate in stage between those shown in Plates 3 d and e) which is still producing primary panicle branch buds. In Plate 3i, the two glumes and the lowest two lemmas have been initiated, and, on the lefthand side, the third lemma is just appearing. In Plate 3J (an enlarged view of the tip of Plate 3f) the two glumes and all four lemmas have been initiated, whilst the two stamens

4 THE NEW PHYTOLOGIST, 59, i PLATE 3 SHARMAS INFLORESCENCE AND SPIKELETS OF AXTHOXANTHUM ODORATLM (Facing p. 62)

5 THE NEW PHYTOLOGIST, 59, i PLATK 4 SIL\RM.\N INFLORESCEN('F AND SPJKICLETS OF AXTII(JX,ANTHUI\I ODORATUM (Facing p. 63)

6 Inflorescence and spikelets of Anthoxanthum odoratum 63 are now being initiated and are seen as two rounded protuberances. From this photograph, together with those in Plate 4 a and b, it will be seen that the stamen initials do not bear any resemblance to leaf primordia but indeed appear to be small bud primordia. Furthermore, one cannot resist the impression that they have arisen in the axils of the last two lemmas, see Fig. 2 a, b and c, which are line drawings of the photographs shown in Plate 3 i and j and Plate 4b. The development of the anther region of the stamen is brought about by the primordium becoming at first almost spherical, as in Plate 4b, and then more elongated and dimpled, as in Plate 4c. With the further growth of the four archesporial regions within, the anther assumes the characteristic shape seen in Plate 4 e, f, h-1. Fig, 2 a, b and c, outline drawings of the spikelets shown in Plate 3 i and j and Plate 4b respectively. For further explanation, see text, G, glume; L, lemnna; S, stamen. The carpel arises soon after the two stamens, appearing as a single couar-like structure, as in Plate 4 d and f. Plate 4g is taken, looking along the plane of the two stamens, so that one is looking directly at the back of the young carpel which is encircling the apex tip seen as a dome in the centre. The mid-region (i.e. the region where it was first initiated) is inserted slightly lower on the apex than the two edges as they become initiated successively further and further round the apex, leading to the appearance seen in Plate 4 e, f, h and i. Except perhaps for this slightly tilted insertion, which may be due in part to the slowing down of the growth of the apex tip, it is evident that the young carpel arises in exactly the same way as a single leaf primordium. There is no suggestion that three primordia are involved, as would perhaps be expected by the hypothetical tricarpellary origin of the grass gynaeceum. Furthermore, it is initiated directly above the third lemma and so continues the strictly alternate sequence of the lemmas and glumes below. Plate 3. Anthoxanthum odoratum. Scale in d applies to a-g and = 0,5 mm. Scale in h applies to h-j and = 0,25 mm, a, apex of a vegetative shoot; b, the same apex with a further leaf primordium removed; c, apex at the end of the vegetative phase; d, young inflorescence with prinmary panicle buds; e, older inflorescence, still growing at the tip and showing the repeated branching of the panicle buds; f, inflorescence (rather less vigorous than the one shown in e), showing the differentiation of the terminal spikelet; g, part of an older inflorescence, showing the spikelets, each with a pair of stamens and single LEGENDS FOR PLATES 3 and 4 carpel primordium; h, tip of a young inflorescence still initiating primary panicle buds; 1, development of glumes and initiation of lemmas; j, initiation of the two stamen primordia, Plate 4, Anthoxanthum odoratum. Scale in g applies to all figures and = 0,25 mm, a-c, stages in the development of the two stamens; d-i, various views of young spikelets showing the two stamens, the carpel primordium and the tip of the spikelet axis; j-1, stages in the closure of the carpe! and the initiation of the stigmas.

7 64 B. C. SHARMAN This sequence of events takes place at the ends of all the panicle branches; Plate 3g shows part of an inflorescence at the stage when the carpels are developing at the ends of the spikelet tips. Plate 4 j-1 show the early stages in the development of the two stigmas during the time when further growth of the carpel leads to its elosure over the apex tip, which then becomes transformed into the single ovule. CONCLUSIONS Thus in Anthoxanthum odoratum L., the spikelets are composed of two glumes, four lemmas and a single carpel, all of which arise in a strictly alternate sequence and each of which appears to originate as a normal leaf primordiuni. The two stamens are borne in the same plane as the glumes, ete., and in their early stage not only resemble bud primordia but even appear to be the axillary buds of the last two lemmas. This suggests that the relationship of the spikelet parts might perhaps best be represented as in Fig. IC. All the spikelets develop in the same way and the tip of the inflorescence itself becomes transformed into a single terminal spikelet, so that the carpel, lemmas and glumes of this spikelet continue the distiehous sequence of the collar leaf primordia subtending the panicle braneh buds and the vegetative leaves below the inflorescence. REFERENCE, B. C. (1947). The biology and developmental morphology of the shoot apex in the Gramineae. Neiv PhytiiL, 46, 20.

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