Vascular Anatomy of New Zealand Species of Gunnera
[Read before the Otago Branch, June 8, 1943; received by the Editor, June 11, 1943; issued separately, December, 1943.]
It is known that Gunnera, as well as one or two other unrelated Angiosperm genera, shows an anomolous vascular structure commonly termed “polystely.” Stems show a number of vascular strands, each surrounded by endodermis and structurally similar to the single condensed stele found in a number of monostelic aquatic Angio-sperms.
The term “polystely” is used here as employed by Scott (1923, p. 176) “in a purely descriptive sense without any implication that each of the several steles is to be regarded as homologous with the single stele of a typical monostelic stem.”
The vascular anatomy of Gunnera has been the subject of investigation by several European botanists, but unfortunately their published accounts have not been available. Solereder (1908) has briefly summarised their results, which indicate that only incomplete material of a limited number of New Zealand species has previously been examined, and that ignorance of the habit of the plants has caused certain misinterpretations of their stelar anatomy. In the present work, all parts of the plants have been examined. The extent to which the polystelic condition is spread amongst the various plant organs has been observed, the origin, course, and destination of the different “steles” has been worked out, and the monostelic structure of the stolon carefully studied.
Of the eight species and one variety occurring in New Zealand (Cheeseman, 1925, p. 626–630), all except Gunnera densiflora have been available, the material having been collected by the writer and in some cases by others in various parts of New Zealand. Part of the material was fixed for anatomical study, part kept as herbarium material, and in addition each of the species has been grown in the writer's garden under observation. Fixatives used were 70 per cent. alcohol and formalin-acetic-alcohol. The work has been based mainly on parafin-embedded material, serially sectioned and stained chiefly with safranin and light green. Statements made about the anatomy of the various organs are based on the examination of usually at least four objects of each organ in each species, and considerably more where significant variation occurred. For seedling anatomy, material from three species only was used (G. albocarpa, G. strigosa, and G. hamiltoni), a total of eight series of sections, each of a seedling axis, being employed. All drawings except figures 1 and 2 were made with the aid of a camera lucida or projection apparatus.
This paper represents part of a thesis presented for the degree of M.Sc. in the University of New Zealand. The work was carried out in the Department of Botany of the University of Otago, and the writer wishes to express her thanks to Dr. Holloway for suggesting
the subject and for much useful advice and kindly encouragement throughout. Thanks are also due to those who assisted in the collecting of material and in other ways, and to the Dominion Museum for the loan of herbarium specimens.
The New Zealand Species: Their General Morphology and Habitats.
The New Zealand species, unlike those of South America and Hawaii, are all small herbs, Gunnera hamiltoni (fig. 1, natural size) being the stoutest of them. In each, the stem is represented by two distinct organs, a stout erect shoot and a horizontal stolon. The erect shoot is usually not more than a centimetre in length, the leaf bases closely overlapping on it; but with burying it may elongate considerably. At the apex of a young stolon occur two or more scale leaves (fig. 1, sc.). These enclose the turning-up rudiment of the next erect shoot; and, arising from this, in the axil of one or more of the scale-leaves, can already be distinguished the beginnings of new stolon buds. Thus a stolon originates as a lateral bud in the axil of an erect shoot leaf, and the erect shoot itself is actually the turned-up apex of an elongated stolon.
The leafy shoot of the New Zealand species morphologically corresponds to the stout procumbent stems of the large South American species G. manicata and G. chilensis, and the Hawaiian G. petaloidea. The S. American G. macrophylla, of intermediate size, has however an erect stem a few cms. thick from whose leaf axils stolons arise (Skottsberg, 1929), thus corresponding in habit with the small New Zealand species.
From the slender tap-root of the seedling arises an erect-shoot axis, from which adventitious roots soon develop. In the axils of the first few foliage leaves no buds appear. Then, several months from the time of the germination of the seed, stolons develop in the axils of (usually) two leaves. Each quickly elongates, and a new shoot turns up from its apex. Fig. 2 is a plan of a seedling 17 months after germination, each circle representing an erect shoot.
Conspicuous on the axis of seedlings in each species examined (namely, those sectioned and G. monoica) are curious structures termed here “rosette-organs,” which alternate regularly with foliage leaves. They early disintegrate, sections showing that Nostoc cells are entering at their site. The algae, in dense clumps or filaments, are placed intercellularly near the periphery of the stem and intracellularly further in. Similar but more elaborate “rosette-organs” (Fig. 3), resembling some mucilage glands figured for G. macrophylla and G. scabra (Solereder, 1908, p. 337), occur on young erect shoots at stolon apices of older plants, these likewise being subsequently invaded by Nostoc colonies.
All the New Zealand species are moisture-loving. Gunnera monoica, G. monoica var. albocarpa and G. strigosa frequent shady forest-margin banks, wet tussock slopes and sometimes swamps. The latter two habitats also find G. mixta and G. prorepens, both of which differ from the previous species in having larger, blacker leaves with laminae longer than broad. G. dentata grows along lake-margins
Fig. 2.—Plan of 17-month plant, G. albocarpa.
Fig. 3.—Mucilage gland at stolon apex, G. hamiltoni, x 93.
Fig. 4.—T.S. endodermis. G. dentata root, x 970.
Fig. 5—T.S. axis of 5-leaf seedling of G. strigosa, just above cotyledons, x 220.
Fig. 6.—G. albocarpa seedling, x 16. r.o., rosette organ; cots., cotyledons.
Fig. 7.—T.S. axis of young seedling of G. albocarpa just below cotyledons, showing strand leaving monostele for “rosette-organ.” x 30.
Figs. 8 and 9.—Sections through axis of a 5-leaf G. strigosa seedling, showing simple trace to second and third toliage leaf (L2 and L3) and more complex trace to fourth foliage leaf (L4), the monostele changing to polystele. N. Nostoc colonies. x 26.
Fig. 10.—Yasculan cylinder of portion of G. hamiltoni short erect shoot, x 27. L1-L4, leaf trace systems to successive foliage leaves; st., stolon stele; p., strand to inflorescence peduncle; N.S., strands to Nostoc colonies; r., roots.
Fig. 11.—T.S. simplest type of stolon stele in G. prorcpeus, x 283.
Fig. 12.—T.S. single strand in G. hamiltoni peduncle, x 150.
Figs. 13, 14, 15.—Sections through node of G hamiltoni, x 10. 13—adventitious roots (t.) and traces to dorsal and ventral scales (d.sc. and v.sc.) break stolon stele. N. Nostoc colony; e l.s., external lateral strand to scale. 14—Trace to lateral sealy leaf (sc.L.) leaves left side of stelar cylinder, while at sides of gap are new stolon-protrusions (s.s.). Gap at right for lateral scale trace. 15—New stolon steles (st.) have formed at left and right, by fusions of stelar protrusions in scale-leaf axils; remaining stelar cylinder broken into polystele as leaf trace systems develop.
Fig. 16.—T.S. portion of G. striqosa root stele, showing phloem bay with radially arranged elements. x 285.
Figs. 17–20.—T.S. petioles of G. manicata, G. capensis, G. hamiltoni and G. arenaria, all x 4.
Fig. 21.—T.S. G. prorepens stolon stele with external and internal xylem (outlined) and phloem (dotted). x 70.
Fig. 23.—T.S. portion of an erect shoot stele in G hamiltoni, showing secondary xylem and phloem. (Note nuclei in several cambial cells). x 300.
and river-beds, and may be submerged for weeks at a time. G. arenaria and the stout G. hamiltoni occur in moist hollows among coastal sanddunes, the latter being endemic to a few localities on the shores of Foveaux Strait.
Histology of Endodermis, Phloem, and Central Ground Tissue.
For an accurate understanding of the structure of the steles, the recognition of these tissues is important.
Endodermis: Casparian bands were the criterion taken (Fig. 4). Whereas in radial endodermal walls cut precisely transversely a Casparian dot was sometimes difficult to detect, on slightly oblique walls and end walls the band could be seen as a narrow red-stained refringent ribbon on the otherwise green wall. Sometimes it was strongly stained and with sharply marked edges; at others, its margins were less distinct, suggesting in extreme cases of this that the endodermis was then vestigial. The band is placed, on the cell-wall, closer to the vascular strand than to the cortex (Fig. 5). On the cortex side of it, the wall shows in transverse section an especially thin region (Fig. 4). Here the wall readily bends and sometimes breaks. Such bends in walls, when associated with occasional Casparian bands, serve as a useful means of determining the course of an endodermis. Alone, however, this does not appear a safe criterion, as around strands in leaf blades similar bent walls occurred, although Casparian bands were in no case detected.
Phloem: In transverse sections the phloem elements differ from adjacent pericycle or internal parenchyma cells in the following features: Small size (an entire group often being of no greater diameter than a single nearby parenchyma cell); walls always unthickened; presence (not invariable) of companion cells; frequent arrangement in radial rows. Sieve-tubes in longitudinal section show transverse or but shallowly oblique end-walls, each of which has a single sieve-plate perforated by numerous pores. A callus was in no case observed. On lateral walls were numerous sieve-areas, each with several specks (either actual or vestigial perforations). The sieve-tube contents frequently appear in the form of the characteristic “slime-funnel.”
Central Ground Tissue: This tissue, internal to the steles in the erect shoot and peduncle and sometimes occurring in the centre of a stolon stele, consists of large cells, somewhat elongated, parenchymatous, and with intercellular spaces. Thick-walled stone-cells may occur, whose end-walls are lignified towards their periphery and possess several large pits. Pale, unstained areas sometimes occur on the lateral walls, similar to sieve-tube lattices except for the absence of the minute perforations the latter show. In each of these features the central ground tissue is similar to the cortex, from which it seems histologically indistinguishable.
Seedling: In the young seedling, the slender stele from the diarch tap-root, on passing up through the hypocotyl, expands in diameter and becomes elliptical in section. At this point from each side, at
right-angles to the cotyledons, a very slender vascular strand leaves the monostele and runs to the rosette-organ (Fig. 7). This strand usually has one or more scalariform tracheids, and is surrounded by an endodermis with Casparian bands. Shortly above, a stouter strand passes to each cotyledon, leaving an irregular monostele. From this, single traces soon leave for first, second and third foliage leaves. Each takes a progressively larger “bite” out of the stele as it goes, so that the stele left after the third is shaped in transverse section like a thick horse-shoe (Fig. 8). When the stele closes again over this leaf-gap, it is by the meeting of the edges only, so that the seedling's solid core of vascular tissue is thus changed to a hollow cylinder, in which a few large central ground tissue cells are enclosed. In the meantime, the trace to the fourth foliage leaf is being formed. It receives not only a stout median strand which causes a gap in the stelar tube, but also two internal lateral strands, coming from the inner margins of adjacent stelar tissue (Fig. 9). This triple origin of the leaf trace, with or without accessory complications, is the typical arrangement in the adult plant of each New Zealand species, and breaks the condensed monostele of the seedling into the “polystelic” condition of the adult erect shoot. The three strands fuse as they pass through the foliar gap, crossing the cortex as a single, concentric “stele.”
From now on the structure of the seedling is merely a small-scale version of the adult erect shoot; so the latter will be described.
Adult plant: Figure 10 shows a model of a portion of an erect shoot, built up from serial sections. It is of a short stem of Gunnera hamiltoni, the New Zealand species in which the stelar structure is best developed. Leaf trace L.4 shows the fusion of the median leaf strand, arising in the gap, with the two main and one small accessory internal lateral strands, the resulting trace passing out through the leaf-gap. In L.1, L.2 and L.3, as this trace crosses the cortex, it gives off lateral branches. In each case one of these fuses with an asymmetrical external lateral strand, which leaves the outer surface of the stelar tube without causing a gap. Occasionally there is an external lateral trace on the other side as well.
In the other species, the leaf trace similarly arises from a median and two internal lateral strands. But external lateral strands were in no cases seen. The trace passes up the petiole as a single concentric strand surrounded by endodermis in G. dentata, G. arenaria, and almost invariably in G. prorepens. In G. monoica, albocarpa and strigosa it sometimes remains single, sometimes gives off a pair of lateral branches.
Alternating with the leaf traces, there leave the outer surface of the stelar tube extremely slender vascular strands. These run to the Nostoc colonies which replace the rosette-organs. Usually a pair passes to each colony, branching or remaining single (Fig. 10, N.s.). They terminate blindly among the Nostoc-laden cells in the cortex. with slightly bulbous ends surrounded by endodermis.
Root steles arise from the outer surface of the stelar tube without causing gaps, and pass at varied angles through the cortex and out (Fig. 10, r). Pentarch roots are commonest, but this is variable for
each species. G. hamiltoni shows typically from 8 to 12 protoxylem groups in the root stele.
Most commonly the stelar tube of the erect shoot closes above a foliar gap without formation of a bud. But at times from the sides of a gap arise a pair of stelar protrusions which extend out into the cortex. These may fuse to form a strand to an inflorescence peduncle (Fig. 10, p.), this subsequently breaking up into several concentric strands; or they may constitute a new stolon bud.
To form a stolon-bud, in G. hamiltoni, each of the pair of stelar protrusions runs out as a strand sausage-shaped in section, the two presently fusing at top and bottom to form a stolon monostele, in the middle of which is central ground tissue continuous with that inside the axis cylinder (Fig. 10, st.). The stolon stele thus constituted is an elliptical tube of vascular tissue, separated outside from stelar cortex by an external endodermis, and inside from central ground tissue by internal endodermis. Sometimes a slender additional strand may leave the margin of the leaf gap and run through inside the stolon tube, separated throughout from the central ground tissue by an endodermis of its own.
Whereas Gunnera hamiltoni stolon steles always surround a considerable core of central ground tissue, the stolons of other species arise from more slender protrusions, which sometimes enclose a little central ground tissue but more commonly fuse completely. The stele is then elliptical in transverse section and with external endodermis only. An interesting variation of this is found within Gunnera arenaria. Sometimes, as in the other species, the two protrusions from over a leaf-gap fuse completely to give the typical elliptical stele. At others, they partially fuse, so that the stele running through the stolon is dumb-bell shaped in transverse section; and occasionally they do not fuse at all, passing through the stolon side by side as two separate steles each surrounded by its own endodermis. This was the only exception found to the plan that in the New Zealand species the stolon steles are monostelic.
The stolen stele, once constituted, runs through the length of an internode without change. As it approaches the next node—i.e., the next erect shoot, root steles enter it and its dorsal and ventral points break to give off single traces to dorsal and ventral scale leaves (Fig. 13). Then what was the stolon stele broadens laterally, and becomes broken up by the foliar gaps of traces going to lateral scale leaves (in G. hamiltoni) or the lowest foliage leaves (in the other species) (Fig. 14). Over these two lateral gaps, stelar protrusions arise in the leaf axils and pass out to fuse as new stolon elements (Fig. 15, st.). One of these soon elongates as a new stolon internode. The other more frequently remains dormant. In between them the remainder of the stele is quickly broken up into the “polystelic” condition of the erect shoot vascular tube by the formation of further foliar traces and gaps.
At a stolon apex, enclosed by scale leaves, not only is all of the above shown in embryonic form, but one of the stolon buds may have a second rudimentary erect shoot, again with axillary rudiments of further stolons.
Arrangement of Vascular Tissues.
A single section through the axis of an elongated erect shoot of any Gunnera species shows a number of “steles,” each surrounded by an endodermis. Among the New Zealand species, a G. hamiltoni stem will show about five “steles” belonging to the main vascular tube, and about 30 accessory strands belonging to leaf-trace systems, Nostoc colonies and roots; whereas a stem section of any of the other species will show about three main and eight accessory strands. This stands in sharp contrast to the condition pertaining in large extra-New Zealand species. The S. African G. capense, of intermediate size, showed in an example studied a total of 59 strands; while the large G. manicata shows many hundreds in one section. And whereas none of the New Zealand petioles have more than three strands, G. capense has about 16, and G. manicata, in a small leaf, 103 (Figs. 17–20, all same scale).
Each “stele” of the erect shoot main vascular tube (there are five of these in Fig. 10) of any New Zealand species is of concentric structure. In each of these steles, passing from the external to the internal endodermis (Fig. 22) there occurs: pericycle, phloem, metaxylem, protoxylem, unlignified tissue; and then internal protoxylem, metaxylem, phloem and pericycle, each tissue thus duplicated in reverse order. One feature deserves special comment. Both phloem and metaxylem elements are repeatedly arranged in short radial rows—a condition observable in each species and best shown in G. hamiltoni (Figs. 22 and 23). Likewise in radial longitudinal sections one finds several phloem elements, and then two or three protoplasm-filled cells between the phloem and the xylem, all of the same length. These protoplasm-filled cells seem difficult to interpret as other than a more or less vestigial cambium. And in the axis of seedlings, whereas poor differentiation of phloem makes details of the tissue arrangement difficult to sort out, it appears significant that in most specimens examined there was a distinct radial arrangement in the phloem and xylem both in the hypocotyl and just above the cotyledon traces (Fig. 5).
Leaf-trace strands are also concentric, but here a radial arrangement of elements is not apparent. The Nostoc strands are too slender for their arrangement of xylem and phloem to be significant.
Inflorescence peduncles contain usually 10 to 12 strands in G. hamiltoni, 3 or 4 in the other species. Each strand (Fig. 12) is again concentric, and almost identical with a petiolar strand. Sometimes they are surrounded by endodermis with clear Casparian bands, at others with diffuse Casparian bands; while in other cases again, within one species, careful examination of side and end walls of cells failed to detect a Casparian band at all.
The roots in most of the species consist probably entirely of primary xylem and phloem. In Gunnera strigosa and G. hamiltoni, however, phloem bays often have beneath them several radial rows of cells, the lowermost of which are sometimes slightly lignified (Fig. 16).
That the stolon of G. hamiltoni has internal as well as external endodermis has already been stated. In passing inwards from the latter there occur, as in the erect shoot steles, all the elements of a
normal bundle in the usual and then in reverse order (Fig. 24). And amongst both internal and external phloem there occur frequent groups of cells in radial rows. In Gunnera prorepens, the stolons examined showed a great variety of stelar types which can be arranged in a series showing progressive simplification—viz., a type with all tissues represented twice, as in G. hamiltoni (Fig. 24, inset): steles with internal xylem and phloem but no internal endodermis (Fig. 21); some with xylem as the only internal tissue; and a few with but a single ring of xylem and phloem surrounded by external endodermis, these last being structurally identical with the single condensed stele of certain aquatic Monocotyledons (Fig. 11). The measuring and classifying of 26 G. prorepens stolon steles into the above divisions showed that there was a close relationship between their size and complexity, only occasional slender and starved-looking specimens showing the simplest type of stele, presumably through reduction. The other species showed similar though less extensive variation in their stolon structure.
A brief examination of a stem and leaf strands of G. manicata and G. capense revealed no indication of the radial arrangement of vascular tissues which is present in the New Zealand species.
The genus Gunnera is known to possess a number of specialised features. There is considerable floral reduction. The embryo-sac does not show the usual 8-nuclear condition. The vascular system is highly anomalous. The genus is widely scattered throughout the Southern hemisphere and in the Pacific Ocean, this suggesting considerable antiquity.
Although these specialised features seem characteristic of the genus as a whole, there is among the species wide variation in morphology and in details of the vascular anatomy. It would seem that the genus can be divided into large species, such as G. manicata, with its single type of procumbent and elaborately polystelic stem, and small species such as those of New Zealand with a mildly polystelic erect shoot, a monostelic stolon, and vestiges of cambial activity. But that such a division is not sharp is shown by the fact that, of two species of intermediate size, the S. African G. capense has the habit of the large species, whereas the stouter G. macrophylla in S. America is stoloniferous. (Skottsberg, 1929).
The large species with their huge herbaceous habit, their great number of vascular strands, and their many forms of mucilage glands and scale leaves, are clearly highly specialised. By contrast the simpler morphology of the N.Z. species suggests a more primitive condition. This is supported by their less elaborate type of stem polystely, in which a tube of vascular tissue has gaps made in its sides by outpassing leaf traces. It is to be noticed that all strands arise from this continuous hollow cylinder, and that none originate or end blindly in its zone. Hence the “polystelic” state suggested by isolated sections resolves itself, when surveyed three-dimensionally, into a hollow monostele with foliar gaps and branches. When added to this it is seen that cambial activity is by no means lost, the condition approaches more closely than ever to the normal dicotyledonous
arrangement. But in the presence of both internal phloem and internal xylem, orientated inversely and bounded by an internal endodermis, the difference from a normal dicotyledonous stem is apparent.
Whether or not the internal xylem, phloem and endodermis of the Gunnera axis, together with central ground tissue, is to be regarded as a phylogenetic inflowing of tissue at the large leaf gaps, associated with the inpassing of the lateral strands of traces until these appear as internal lateral strands, is a matter passing largely into the realm of speculation, though not entirely lacking support in the seedlings. And why, by this or other method, the leaf gaps should break the original cylinder into an anastomosing system of concentric strands, is again obscure. Scott (1890–1, p. 516) and Arber (1920, pp. 180–182 and p. 346) point out that both Gunnera and Auricula are closely allied to aquatic plants, of reduced vascular structure and without secondary growth; the suggestion being that these two genera have evolved from aquatic ancestors and, having lost cambial activity, have independently achieved the elaboration of their vascular systems necessary for a large terrestrial habit by developing the polystelic condition. If this were so, the New Zealand species would appear to be an intermediate stage, in that cambial activity is not completely lost, the vascular cylinder is only mildly broken up, and a large herbaceous habit has not been attained.
One might be tempted to look to the stolon rather than the erect shoot in such speculations, as this in its simplest condition is closely comparable structurally with the condensed stele of Myriophyllum and other aquatic angiosperms. But the stolon arises in development secondarily from the erect shoot, as a specialised form of bud; its degree of structural complexity appearing to depend largely on the size and shape of the two outgrowths from the axis cylinder which fuse to give rise to it. Hence the simplest type of stolon stele in G. prorepens seems better regarded as a half-starved abnormality than as the basal form of stele for the genus.
That the N.Z. species are more primitive than the huge specialised herbs of S. America, Juan Fernandez, Hawaii, Java, and S. Africa is a hypothesis supported by much evidence. But, in the absence of a knowledge of the anatomy of the S. American small species, and of whether these or any of the larger ones show vestigial cambium, speculations as to where the genus originated cannot be soundly based.
Abber, A., 1920. Water Plants, Camb. Univ. Press.
Cheeseman, T. F., 1925. Manual of the New Zealand Flora. Wellington, Govt. Printer, N.Z.
Scott, D. H., 1923. Studies in Fossil Botany, vol. II. A. & C. Black, Ltd.
— 1890-91. Origin of Polystely in Dicotyledons. Ann. Bot. vol. V, pp. 514–517.
Skottsberg, C., 1929. Bermerkungen über die Morphologie von Gunnera macrophylla Bl. Meddel. Göteborgs Bot. Trädgard, V., pp. 115–126.
Solereder, H., 1908. Systematic Anatomy of Dicotyledons, vols. I and II. English Ed., Clarendon Press.