Reaction Tissues in Gnetum Gnemon a Preliminary

Home , Gnetum, Gnetum africanum, Gnetum gnemon

IAWA Journal, Vol. 22 (4), 2001: 401–413

REACTION TISSUES IN GNETUM GNEMON

A PRELIMINARY REPORT by P. B. Tomlinson Harvard Forest, Harvard University, Petersham, MA 01366, USA, and National Tropical Botanical Garden, 3530 Papalina Road, Kalaheo, HI 96741, USA

SUMMARY

Gnetum gnemon exhibits Rouxʼs model of tree architecture, with clear differentiation of orthotropic from plagiotropic axes. All axes have similar anatomy and react to displacement in the same way. Secondary xylem of displaced stems shows little eccentricity of development and no reaction anatomy. In contrast, there is considerable eccentricity in extra-xylary tissue involving both primary and secondary production of apparent tension-wood fibres (gelatinous fibres) of three main kinds. Narrow primary fibres occur concentrically in all axes in the outer cortex as a normal developmental feature. In displaced axes gelatinous fibres are developed abundantly and eccentrically on the topographically upper side, from pre-existing and previously undetermined primary cortical cells. They are wide with lamellate cell walls. In addition narrow secondary phloem fibres are also differentiated abundantly and eccentrically on the upper side of displaced axes. These gelatinous fibres are narrow and without obviously lamellate cell walls. Eccentric gelatinous fibres thus occupy a position that suggests they have the function of tension wood fibres as found in angiosperms. This may be the first report in a gymnosperm of fibres with tension capability. Gnetum gne-mon thus exhibits reaction tissues of unique types, which are neither gymnospermous nor angiospermous. Reaction tissues seem important in maintaining the distinctive architecture of the tree. Key words: Gnetales, Gnetum, reaction fibres, tree architecture, gelatinous fibres, stem anatomy.

INTRODUCTION

This article reports a unique reaction anatomy in the Gnetales, where reaction anatomy may be defined as secondary structural responses to external stimuli, particularly those involving displacement of axes. Reaction tissue (ʻReaktionsgewebeʼ of German authors) is usually defined as the relevant tissue that brings about either secondary re-orientation of mature plant organs or maintains organs in a fixed position relative to gravity by exerting some internal applied force. Reaction tissue is most familiar in the structural changes in secondary xylem of conifers and hardwoods, the tissue be- ing referred to collectively as ʻreaction woodʼ. Reaction wood includes specialized

Downloaded from Brill.com09/24/2021 12:45:18PM via free access 402 IAWA Journal, Vol. 22 (4), 2001 Tomlinson — Reaction fibres in Gnetum bark 403 cells derived from cambial fusiform initials after the stimulus is received. Conifers and hardwoods are strongly contrasted in the type of reaction wood they produce and the way in which a mechanical force is generated. Compression wood occurs in conifers and Ginkgo (Timell 1980) and consists of modified tracheids (compression wood tracheids) usually developed eccentrically on the lower surface of a leaning stem or branch. The force generated, apparently by extension of cells, is positive, i.e., a ʻpushʼ. In contrast, hardwoods develop tension wood eccentrically, usually on the upper surface of leaning axes. The force generated, apparently by contraction of cells, is nega- tive, i.e., a ʻpullʼ and the tissue and cells responsible are termed tension wood and tension wood fibres, respectively. Tension wood fibres, in the absence of certain knowl- edge about their mechanical function, are often termed ʻgelatinous fibresʼ, from their characteristic highly hydrated, refractive and unlignified secondary cell walls, a con- vention followed here. It should be emphasized that reaction tissues, in an ecological context, can play many roles in the dynamics of woody plant organs, although this is often overlooked in the wood anatomical literature. For example, it can occur in leaves (Scurfield 1964; Staff 1974; Sperry 1982) of both dicotyledons and monocotyledons. It is responsible for the contraction of pendulous aerial roots in Ficus when they become rooted dis- tally, an unusual circumstance because the reaction tissue is developed concentrically (Zimmermann et al. 1964). It apparently plays an essential function in the early estab- lishment of the viviparous seedlings of mangrove Rhizophoraceae (Tomlinson & Cox 2000) and has been implicated in the maintenance of overall tree architecture (e.g. Fisher & Stevenson 1981). Reaction wood can be extensively developed in seedlings (Scurfield & Wardrop 1962). An interesting evolutionary question is to consider the Gnetales in terms of reaction tissue. In a typological sense this group of plants stand intermediate between gymnosperms and angiosperms. Despite recent vicissitudes about the phylogenetic status of the Gnetales (e.g., Donoghue & Doyle 2000) recent studies show them to be most closely related to the conifers. However, as is well known, their secondary xylem includes well-developed vessels, with mainly simple but sometimes forami- nate perforation plates (Fisher & Ewers 1995). This raises the question addressed here: “Are the Gnetales gymnosperm- or angiosperm-like in their reaction anatomy?” The preliminary observations reported here show, at least in Gnetum gnemon, that they exhibit reaction tissues of unique types, neither gymnospermous nor angiospermous.

MATERIALS AND METHODS

Gnetum gnemon L. is a small dioecious tree widely cultivated in Southeast Asia as a kampong (village) plant for its edible leaves, seeds and for fibre (Burkill 1966), with a natural distribution in the Malay Archipelago from Malaysia to Papua New Guinea. Material was derived from a small population of male and female trees, appropriately cultivated at ʻThe Kampongʼ of the National Tropical Botanical Garden, Coconut Grove, Miami, Florida, USA. Trees reiterate trunk axes abundantly to provide an am- ple supply of stems for destructive sampling. For experimental procedures orthotropic

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axes 3–5 cm diameter were tied down by nylon string, the bark protected by bands of bicycle tire inner tube. A series of plagiotropic axes were similarly bent below the horizontal (i.e., below their natural orientation). In both experiments this introduced abnormal eccentric mechanical stress. After 6 months (June 2000 to January 2001) the axes were cut and an approximate map made of their shape, dimensions and internode number with lengths. To preserve the manipulated orientation of the stems a shallow groove was scored along their upper surface at the time they were collected. The groove appears as a shallow notch in subsequent transverse sections, indicating the upper sector of the axis (e.g. Fig. 10 n). The results section deals almost exclu- sively with orthotropic axes, the greater complexity of plagiotropic axes is only com- mented upon briefly. Transverse sections were cut without embedding from the middle of each internode, starting with the youngest, using a Reichert sliding microtome. Section thick- ness varied from 40 μm to 100 μm since it was necessary to produce an overall view without regard to section quality, as appears clear from the illustrations of thicker axes (Fig. 11–16). Longitudinal sections of a few axes were prepared in a similar way. Most sections were stained in 0.1% aqueous toluidine blue, washed in tap water and mounted in glycerine:water, 1 : 1 by volume, making semi-permanent preparations. For histological purposes sections were stained in 95% alcoholic phlorogluci- nol and concentrated (38%) hydrochloric acid, 1 : 1 by volume as a test for lignin. Starch distribution was determined by mounting sections in aqueous iodine/potassium iodide (Lugolʼs iodine). For a study of cell types material was macerated by boiling slivers of tissue for three minutes in 10% aqueous potassium hydroxide followed, after repeated wash- ing, by 20% aqueous chromic acid (chromium trioxide). After 15–20 minutes the material was sufficiently soft to be rinsed and teased apart on slides in the glycerine mountant. Material was examined on an Olympus S2H compound microscope, photographed on Ektachrome and the colour transparencies converted to illustrative plates in the computer graphic laboratory at Harvard Forest. Cell wall features were enhanced, where necessary, by use of polarizing optics with a supplementary colour plate. Some material was embedded in ʻParaplastʼ and sectioned on a rotary microtome in the usual way, but gave poor initial results because of the high crystal content of young tissues.

RESULTS Architecture Gnetum gnemon provides a precise example of Rouxʼs model in the Hallé-Oldeman system of tree architectural models (Hallé & Oldeman 1970; Hallé et al. 1978). Phyl- lotaxis is decussate throughout. Orthotropic (trunk) axes (O in Fig. 1, 3 & 4), with radial symmetry, have continuous branching with a branch pair produced by syllepsis at each node so that four orthostichies remain conspicuous (Fig. 1). Plagiotropic axes (P in Fig. 1, 3 & 4), initially close to the vertical, become horizontal with age. Dorsi- ventrality of each branch complex is established as each leaf pair rotates by twisting

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Fig. 1– 4. Gnetum gnemon. Habit. – 1: View from the ground up the trunk (orthotropic axis, O) of a tree c. 6 m high, with orthostichies of plagiotropic branches (P) evident. – 2: Reiterated sapling with continuous branching (Rouxʼs model). – 3: Detail of third node of an orthotropic axis (O) to show a pair of sylleptic plagiotropic branches (P). – 4: Older node from a bent orthotropic axis (O); original sylleptic branches (P) retain horizontal orientation; reiterated orthotropic axis (Oʼ) comes from previously dormant bud. — Scale bar for 3 & 4 = 2 cm. of the petiole into a horizontal plane. There is no rotation of the internode. Dorsi- ventrality is maintained because the few higher order branches are in the horizontal plane, with reproductive axes (strobili) the ultimate branch orders. A sapling brought into the open shows the numerous and always relatively slender branches, well ex-

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tended from the trunk (Fig. 2). This delicate appearance is characteristic of trees in their natural habitat in the forest understorey (cf. Hallé & Oldeman 1970: 105). Each leaf on an orthotropic axis subtends a pair of superimposed axillary buds, the upper becoming a sylleptic branch (Fig. 3), the lower persistent as a suppressed and inconspicuous reserve bud. Damage to or manipulation of the axis induces the proleptic development of a reserve bud always as an orthotropic axis which repeats the architecture of the whole tree (cf. Hallé et al. 1978: 57). This contrast in developmental potential of closely juxtaposed buds is common in tropical trees and is shown in Figure 4 in which a displaced trunk axis (O) has produced a single proleptic reiteration (O,) from the node supporting a pair of sylleptic laterals. Reiteration of this kind in Gnetum contributes largely to the development of a denser crown than the primary architecture (Fig. 2) might suggest. Older, but still relatively slender plagiotropic axes eventually abscise to leave a prominent circular scar. The tree is not only distinctive in the highly articulate construction of its axes, but also in their individual development since each internode functions as an autonomous unit, largely completing extension growth before the next youngest internode begins to elongate as reported for the lianescent Gnetum africanum (Miloundama & Mbon 1992). These developmental aspects are relevant to a study of reaction anatomy but are not reported on further in this article.

Primary construction The youngest fully extended internode of any axis includes an approximate cylinder of primary vascular bundles surrounding a very wide medulla (Fig. 5). The medulla is ellipsoidal, the long axis, in the plane of insertion of each leaf pair, rotating through 90° in each internode in relation to the decussate phyllotaxis. The vascular system includes a cylinder of widely separated vascular bundles with normal (i. e., endarch) primary xylem (Fig. 6 & 7). Immediately outside the vascular cylinder is a layer of starch-rich parenchyma (cf. Fig. 9) but most prominent is a discontinuous series of large, thick-walled and lignified astrosclereids (scl in Fig. 6), which essen- tially delimit the stele from the cortex, as does an abrupt change in cell size (cf. co and me in Fig. 6). Similar sclereids are scattered in the medulla and less commonly in the cortex. With age this sclerotic layer is augmented by late differentiating cubical sclereids (brachysclereids) into a more or less continuous sclerotic cylinder. Primary phloem of each vascular bundle consists of radial files of sieve cells and parenchyma, capped by the collapsed remains of the protophloem (Fig. 7). This layer of collapsed cells is described as “obliterated phloem” by Esau (1969, fig. 98). Most of the cortex appears parenchymatous in transverse view (Fig. 6), but in longitudinal sections non-septate laticifers with densely staining granular contents are evident. Ground parenchyma cells of the cortex are relatively short, but in addition to the laticifers there are numerous elongated thin-walled cells which can play a major role in subsequent development. The outer cortex of all axes includes narrow scattered gelatinous fibres, shown to be refractive under polarized light (Fig. 8 gf). These cells seem to be a significant contributor to stem stiffness in the young internode because other mechanical tissues are little developed.

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Fig. 5–10. Gnetum gnemon. All transverse sections of orthotropic shoots. – 5: Primary stem structure. – 6: Detail of vascular cylinder with primary vascular bundles. – 7: Single primary vascular bundle with early development of the interfascicular cambium (cb). – 8: Outer cortex with first-order gelatinous fibres. – 9: Older internode with early secondary xylem; stained with I2KI to show pronounced starch sheath of inner cortex. – 10: Old internode with well- developed secondary xylem and slight eccentric growth, from a bent orthotropic axis. — Let- tering: cb = interfascicular cambium; co = cortex; gf = gelatinous fibres; me = medulla; n = notch for orientation; scl = astrosclereid layer; sec = secondary xylem; st = starch-rich layer of inner cortex. — Scale bars = 1 mm in 5; 200 μm in 6 & 9; 100 μm in 7; 50 μm in 8; 3 mm in 10.

Secondary construction Secondary growth is initiated by tangential longitudinal divisions in interfascicular cells and continuous with the fascicular cambium (Fig. 7 cb). The original broad interfascicular regions persist in the secondary body as broad rays, the width of the xylem bands determined originally by the size of the original primary bundles (Fig. 9). The original starch sheath becomes pronounced, as seen in sections stained with I2KI, marking the inner limit of the cortex (Fig. 9 st), whereas in sections stained for lignin the band of sclereids forming the outer layer of the stele is made prominent. Progressive increase in amount of secondary xylem occurs in each successively older internode. As described by Carlquist (1994) the secondary xylem includes

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tracheids and fibre-tracheids as well as the conspicuous vessels. Axial parenchyma is inconspicuous but can be demonstrated by its starch content. Regular growth rings are not developed.

Xylem eccentricity Secondary xylem in all axes shows very limited eccentricity, some of which is a consequence of the initial eccentric shape of the medulla (cf. Fig. 5 & 10). The ratio of the thicker to the thinner side never exceeds 1.3 : 1 and does not show any consist- ent orientation with gravity. The eccentricity of Figure 10 may be a result of its artificial displacement, the single discontinuity in the wood possibly reflecting a change in- duced by manipulation. Neither tension wood fibres nor compression tracheids are developed. In conclusion, the marked eccentricity developed in coniferous and angiospermous woody axes when displaced does not occur in Gnetum.

Cortical eccentricity In contrast to the xylem, cortical eccentricity occurs in all kinds of axis but is most pronounced in those that have been artificially displaced. Eccentricity is determined not so much by the addition of new tissues as much as by changes in existing tissues, all of which produce gelatinous fibres. These are best observed in part-polarized light, as in Figures 11–13, oriented so that the upper notched side of the axis is placed at the top of the illustration. Figure 11 is of a relatively young axis, with the four compass orientations used in the numerical analyses shown as N, S, E, W. Figure 12 is the same axis shown in Figure 10 with the marked increase of extra-xylary tissue on the notched side made evident. Details of this tissue are shown at higher magnification in Figures 13 and 14. Extra-xylary tissue on the lower (ʻsouthʼ) side of these kinds of section always remains in the parenchymatous state shown in Figure 9. In axes that have responded to displacement in this way three kinds of gelatinous fibres can therefore be recognized. First are the original outer cortical fibres (co 1 in Fig. 14 and 16, cf. Fig. 8); second are the newly and extensively developed middle and inner cortical fibres (co 2 in Fig. 14 and 16), which are much wider than the outer fibres; third are the massive and almost continuous bands of gelatinous fibres in the secondary phloem, which becomes almost occluded (sec pf in Fig. 13, 14 & 15). These fibres in turn are narrow, of about the same dimensions as the outer cortical fibres. The three types of fibre are contrasted in Figures 15 and 16. They may be referred to as first-, second- and third-order fibres. As is typical of gelatinous fibres all these fibre types have a narrow lumen, refractive walls in hydrated preparations, with the cellulosic secondary wall often convo- luted and separated from the lignified primary wall. In the wide fibres of the middle and inner cortex, co 2 in Figures 15 and 16, the secondary wall of the fibres is most obviously multilamellate. The contrast between cortical and secondary phloem gelatinous fibres is shown particularly dramatically in Figure 13 under polarized light at low magnification, since the intervening band of tissue corresponds to the sclerotic/starch layer delimiting the innermost cortex.

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Fig. 11–16. Gnetum gnemon. All transverse sections of orthotropic shoots in part-polarized light (except 14, in normal transmitted light). – 11: Young eccentric axis with gelatinous fibres on upper side indicated by notch. – 12: Same stem as in Fig. 10 to show extensive development of gelatinous fibres. – 13: Outer stem layers in region of notch with abundant gelatinous fibres in cortex (co) and secondary phloem (sec pf). – 14: Details of gelatinous fibre distribution in outer cortex (co 1), inner cortex (co 2) and secondary phloem (sec pf). – 15: Inner cortex to show second-order gelatinous fibres of cortex (co 2) in contrast to gelatinous fibres of

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60 -

50 -

- m.f. North 30 - 40 - - o.f. 20 - 30 - - 10 - 20 - - 0

10 -

West me East 0 - o.f. m.f. sec. xy - 25 20 -

10 - 25 - 20 - -

- South 0 10 - o.f. m.f.

0 o.f. m.f.

Fig. 17. Gnetum gnemon. Counts of distribution of gelatinous fibres in an internode of a displaced orthotropic axis at four compass points where N = North (notched) side uppermost in the bent axis. Counts are from one equivalent microscopic field of view of the cortex and outer secondary phloem. The values (expressed as bar charts) are the average of counts for North and South locations for nine internodes and eight similar East and West counts along the same axis; bar chart for the North values is somewhat foreshortened. — Lettering: co = cortex; me = medulla; m.f. = secondary (reaction) gelatinous fibres of middle and inner cortex; o.f. = primary gelatinous fibres of outer cortex; sec. xy = secondary xylem.

← secondary phloem (sec pf), the inner limit of the cortex represented by brachysclereids (br). – 16: Outer cortex to contrast reaction fibres (gelatinous fibres) of middle cortex (co 2) with primary gelatinous fibres of outer cortex (co 1). — Lettering: br = brachysclereids; co = cortex; co 1 = first order gelatinous fibres of outer cortex; co 2 = second order gelatinous fibres of middle cortex; gf = gelatinous fibres; sec pf = third order gelatinous fibres of secondary phloem. N, S, E and W in Fig. 11 refer to compass orientation in analyses of fibre distribution (cf. Fig. 17).

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Quantification of the results is exemplified in Figure 17, which contrasts the relative abundance of fibres in the outer cortex (o.f.) with numbers from the middle and inner cortex (m.f.) averaged along a single displaced orthotropic axis, with n = 9 internodes. Measurements were taken within one microscopic field of view at constant magnification and corresponding to the four arbitrary compass points, established by the notch (cf. Fig. 11). The number of outer (first-order) fibres is constant for all positions, as expected. The massive development of second-order cortical fibres on the upper (north) side is well contrasted. Their maximum value in one section approached 150. In the thickest, i. e., oldest, axes values in the secondary phloem for the number of third-order gelatinous fibres range from 100 to ~200 in the same field of view, but were not counted precisely.

Origin of cortical fibres The gelatinous fibres of the middle and inner cortex that develop in manipulated axes do not arise de novo as a response to the stimulus of re-orientation. Their precur- sors can be seen in the primary cortex of older axes as wide cells with thin walls and without apparent cell contents. They constitute the bulk of the middle and inner cortex, as in Figures 6 and 9.

Plagiotropic axes The anatomy of plagiotropic axes has not been examined in great detail, but seems identical with those of orthotropic axes. The main difference is that, because of their horizontal orientation, eccentric development of gelatinous fibres in the cortex and secondary phloem usually occurs in axes without any manipulation, but is accentuated when they are artificially displaced. Initial observations indicate that gelatinous fibres may serve in maintaining the orientation of horizontal branches and so support the hypothesis that they function as tension fibres.

DISCUSSION

From these preliminary observations it is clear that in Gnetum gnemon reaction tissue, unlike that of conifers and angiosperms, does not involve secondary xylem. In- stead, gelatinous fibres in three locations can be developed. Initially they are differentiated uniformly in the outer cortex in all axes. Subsequently if an orthotropic axis is displaced from its natural vertical orientation eccentric development of fibres occurs on the upper side in two locations, both in the middle and inner cortex and in the secondary phloem. Plagiotropic axes apparently develop this eccentricity as a normal feature, which becomes accentuated if they are displaced downward. We thus have an association between gelatinous fibres and their distribution that suggests they can function as tension fibres and seem to be an important component of overall architecture. Further experiments are needed to confirm this hypothesis, but the initial observations suggest that Gnetum gnemon could be a useful research object that could elucidate the mechanism of formation and action of gelatinous fibres.

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Carlquist (1994) records gelatinous fibres in the extra-xylary stem tissue ofGnetum gnemon, including the secondary phloem, but without comment on their possible function. Martens (1971) implies the presence of tension wood developed eccentrically on the upper surface of plagiotropic axes, also in G. gnemon, a report not substanti- ated here. He illustrates what are termed “fibres cellulosiques” in G. africanum and which appear to be secondary phloem fibres, and mentions similar cells in the cortex. In her thesis George (1930) makes the curious suggestion that Gnetum gnemon produces both tension and compression wood on opposite sides of the same axis, but without any documentation. Further research on these kinds of cells in the Gnetales generally could be informative. In other Gnetales gelatinous fibres are also recorded in the bark of lianescent species of Gnetum (Carlquist & Robinson 1995; Carlquist 1996a, b). In Welwitschia Carlquist & Gowans (1995) studied mainly the root because of its axially oriented cells. They refer to cells termed “fibresclereids” which are fusiform and have gelatinous walls in which the crystals are embedded. Else- where they refer to gelatinous fibres of the secondary phloem. They suggest that the root may be contractile because of the sinuous course of vascular bundles. In Ephedra, Lev-Yadun (1999) reports eccentricity of the secondary xylem of the scandent species E. campylopoda, but without the formation of reaction wood. He comments that the eccentricity is more conifer-like than angiosperm-like since more secondary xylem is produced on the lower side of the stem. For Ephedra the situation seems complex, but again gelatinous fibres do not occur in the xylem. Carlquist (1992) records as a common condition gelatinous fibres at the periphery of the pith, or in some species as strands throughout the pith. He also records gelatinous fibres in the secondary phloem of two Old World species. In New World species dark-staining fibres in the secondary phloem are said to have a “slightly gelatinous nature” (Carlquist 1989). There is no record of tension wood fibres in conifers, which is not surprising in view of the absence of fibres from coniferous secondary xylem. However, they can produce gelatinous fibres in the secondary phloem, as reported by Liese & Höster (1966). From this we can conclude that in gymnosperms, as represented by conifers and many Gnetales, gelatinous fibres can be present, with some evidence in Gnetum gnemon that they have a role in stem mechanics. Furthermore, Böhlmann (1971) reports putative gelatinous fibres in the bark of Tilia associated with stem eccentricity and a mechanical function (as “Zugbark”) indicating the further diversity of reaction anatomy of angiosperms, but here more directly comparable to the situation in gymnosperms. From this we may conclude that although tension wood fibres are restricted to the angiosperms, gelatinous fibres (which may function as tension fibres) have a more general distribution in seed plants. This raises interesting questions as to the origin and distribution of gelatinous fibres in evolutionary time. We know that they can occur in the three major extent clades conifers, Gnetales and angiosperms, which might suggest their appearance in a shared common ancestor. Subsequent diversification may have produced the unique condition of tension wood in flowering plants.

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These evolutionary questions will clearly be better addressed when more precise information about the ontogeny and function of extra-xylary gelatinous fibres is avail- able. These considerations should be the stimulus for a more precise study of Gnetum gnemon. As a final clue to the function of gelatinous fibres inGnetum one might note that their tensile properties have an application as in their use in bow strings in South- east Asia (Burkill 1966).

ACKNOWLEDGEMENTS

The assistance of Larry Shokman, Superintendent at ʻThe Kampongʼ, in setting up the experiments is appreciated. Jack B. Fisher (Fig. 2) kindly provided laboratory facilities at the Research Center, Fairchild Tropical Garden, Coral Gables, Florida.

REFERENCES

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