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Ovule Structure in Trachyandra Saltii (Asphodelaceae)

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Ovule Structure in Trachyandra Saltii (Asphodelaceae) S. Me. J. Bot. 1997,63(4) 223-226 223 Ovule structure in Trachyandra saltii (Asphodelaceae) E.M.A. Steyn* and G. F. Smith Research Directorate, National Botanical institute, Private Bag X101, Pretoria, 0001 Republic of Sou th Africa Received 2 April 1997; revised 19 May 1997 A scanning electron and light microscopical study of the multi-ovular ovaries of Trachyandra saltii (Baker) Oberm. var. saW; showed thaI the Iccules of open flowers contained bitegmic, arillate ovules, obliquely arranged on axile placentae with the micro pyles of mos! ovules pointing towards the septa. In the uncurved embryo sac the synergids with strongly PAS-positive filiform apparatus and the overgrown central cell nucleus were the most noticeable elements. The antipodals were still present in the chalazal extension of the embryo sac into the hypostase. At anthesis the ovules were hemitropous. It is suggested that, ontogenetically, the development 01 an aril on the ventral side 01 the ovule has prevented anatropy in the Asphodelaceae. Keywords: Aril, Asphodeloideae, embryo sac, ovule morphology, Trachyandra. ·To whom correspondence should be addressed. Introduction Ovules with curved nllcelli and embryo sacs are campylotropous Ovules of nowering plants display, like leaves and flowers, a ri ch (Bouman 1984; Comer 1976). variety in form and structure, depending on the number, shape, Thus, with the exception of orthrotropous ovules which arc position and combination of their component parts. This wealth actinomorphic, ovules of angiosperms develop into decidedly of morphological diversity has led to a profusion of descriptive zygomorphic structures with a si ngle plan e of symmetry, usually terms, th e recognition of an abundance of characters and charac­ referred to as the median sagittal plane. When viewed in this ter states (sensu Von Teichman & Van Wyk 1991) and the classi­ plane in cleared or sagittally sectioncd ovules, thc aforemen­ fication of ovules into various types. Nevertheless, comparative tioned morphological types can be accurately determined. embryologists have repeatedly stressed the conservative nature of Anatropy has generally been accepted as presenting the plesio­ ovular characters and their importance as indicators of taxo­ morphic state in angiosperms, wh ile all othcr forms an~ regarded nomic affinity (e.g. Herr 1984; Von Teichman & Van Wyk 1991; as apomorphies (Dahlgren J991; Bouman 1984). The occurrence Dahlgren 199 1). Like most embryological characters, the form of of such a wide range of ovular types in the Asphodclaceae sug­ the ovule usually displays no phenotypic plasticity, is remarkably gested the possibility that an in-depth study of ovular characters constant within a genus and onen represents a familial character may indicate an evolutionary, intrafamilial pathway to the genera ~ Davis 1966). During a preliminary survey of available embryo­ and thereby may lead towards a bctter understanding of relation­ logical data on the Asphodelaceae (sellslI Dahlgren el al. 1985). ships among the genera included in th e family. we therefore found it rather interesting to note that th e form of However, a more critical evaluation of the literature dealing the ovule seems to vary considerably within this relatively small with ovule morphology in the Asphodclaceae revealed a general family of petaloid monocotyledons. lack of descriptive or graphic detail in the various studies; it was This Old World family includes 16 genera and about 780 spe­ often unclear as to how the authors had come to their conchl­ cies and is generally considered as comprising two easily di stin­ sions. A point in case is an investigation of ovule morphology by guishable subfamilies, the Asphodeloideae and the Alooideae Kativu (1996). No photographic evidence was presented in this (Smith & Van Wyk 1991). Embryologically. all members of the report to substantiate the supposed occurrence of anatropy in sev­ family are characterised by arillate seeds and a tetrahedral eral anthericoid as well as in two asphodi.!loid species, namely arrangement of microspores in the tetrads, resulting from simu l­ Trachyandra safrii (Baker) Oberm. and .Iodrellia lIlacrocarpa taneous microsporogenesis (Dahlgren el al. 1985). As far as Baijnat h. Since flowering material of T. salt;; var. saLti; was read­ ovule morphology is concerned, variations with regard to the ily available from a natural population of plants in the Pretoria shape of the mature embryo sac and the orientation of th e micro­ National Botanical Garden (PNBG), the ovules of this species pyle towards the placenta seemingly occur, since campylotco­ were studied to reveal thl!ir form and internal structure. This pous, anatropous, hemitropuus and atropous (orthotropous) report deals with the findings of the investigation. ovules have been reported (Dahlgren & Clifford 1982; Dahlgren Material and Methods el al. 1985; McNaughton & Robertson 1974; Kativu 1996). Onlogcneticaily. the anatropous stage is attained when the Inflorescences of T. sallii va l". sallii were collected in mid -afternoon ovule primordium curves through 180 ~ so that the ovule becomes when the opening of a few deli cate, gret;:nish-white flowers per inno~ completely inverted and the micropyler-chalazal (longitudinal) rescence indicated the location of the well-camouflaged plants in axis of the nucellus lies parallel to the longitudinal axis of the natural grassland vegetation in the PNBG. A voucher speci men (Steyll 23) of one of the plants has been deposited at the National funicle. In orthcotcopous ovules no curvature occurs, the ovule Herbarium (PRE) in Pretoria. Open tl owers and a few adjacellt 110w­ therefore maintains an upright position and the funicle, chalaza ers with shriveled peta ls were dissected and the ovaries wen: Iixcd and micropyle lie in a direct line. If the curvature towards the ovt!might in a 0.1 M phosphate butTt!red solution (pH 7.4) containing anatropous position is arrested before the primordium has 2.5 % gluteraldehyde and 4% formaldehyde. Conventional methods reached the anatropous condition so that the longitudinal ax is of were used during processi ng for embedding in glycol methac rylate the nucellus lies more or less perpendicular to the longitudinal and eventual sectioning al 2 ~m . All secti ons were stained with the axis of the funicle, the ovule is hemi(ana)tropous. In orthrotro­ periodic acid/Schiff (PAS) reaction and countcrstained with toluid­ pous as well as hemitropous and anatropous ovules, the nucellus ine blue (Feder & O'Brien 1968). cylinder and embryo sac remain straight, i. e. show no curvature. Materi al for a precursory scanning electron (SE) microscopical 224 S. Afr.1. Bot. 1997,63(4) study was fixed as mentioned above, the ovary segments were dehy­ nent ovular parts with regard to each other. According to Johri et drated in a graded .. celone series and criticnl point dried according to al. (1992), the 'micropyle may point upward (epitropous), down­ conyentional procedures. Further dissection, namely the removal of ward (hypotropous) or to a side (lateral: pl eurotropous)'. The the dorsal ovary wall and the septa between the locules, was morc micropyles of the proximal ovules in the locules of Trachyandra readily accomplished after critical point drying. The ovary segments were turned downward, i.e. toward the base of the locule (Figure were then spuuered with gold and viewed in a lSI SX 25 SE micro­ I A) and therefore represented the hypmropous condition, sensu scope . lohri e/ at. (1992). The initiation of the arBs of the ovules, described in embryo­ Observations and Discussion logical literature on the Asphodelaceae as developing 'as an Ovule characters revealed by SE microscopy annular invagination of the distal part of the funicle' (Dahlgren er In the trilocular ovary of Tra chyandra each of the locules con­ al. 1985), was not seen in our material. According to Joshi ta ined six to seven closely packed ovules, alternately arranged in (1937) and McNaughton and Robertson (1974) the aril in Aloe two longitudinal rows on axile placenlae. The ovules were spp. starts to develop during megasporogenesis when the ovule bitegmic and the inner integument, protruding beyond the rim of begins to curve. In open flowers of T. sallii fixed within two the outer integument, formed the micropyle (Figures lA and B). hours after anthesis, the arils were already quite conspicuous With the dorsal ovary wall removed and the ovules seen in dorsal structu res occurring at the ventral side of the ovules (Figures 1A view, it was evident that, as far as the orientation of the ovules in and B). The funicles were short and broadened by the develop­ the Iocules was concerned, these structures were generally ment of the aril which formed a podium on which the ovule obli quely placed with the micropyles of most ovules turned out­ proper was carried and lifted above the placental papillae (Figure ward, i.e. pointed toward the side of the locule (Figure IA). The IB). ovules therefore tended toward pleurotropy. The terms 'pleurot­ In the typical variety of T. saltii the flowers lasted for less than ropous'. 'epitropous' and ' hypotropous' were used by Johri e/ al. 24 hours; at sunrise following the afternoon of anthesis the deli­ (1992) to designate the orientation of ovules in a locule and were cate petals had become shrivelled. The locules of such flowers not applied to indicate the amount of curvature experienced by contained pollen tubes. At the micropylar side of one ovule a pol­ the developing ovule and thus, the final position of the compo- len tube, adcrusted with white deposits, was seen passing over Figure 1 A-D Scanning electron micrographs of ovules of Trachyandra saltii var. saLti;: A. Dorsal view of the obliquely placed ovu les with arils (arrow heads) in one of the locuies after the dorsal wall and septa of the locule were removed. Distal ovu les, i.e. ovules nearest to the style, shown at left in micrograph. Note that di stal and central ovules are pleurotropous, since thei r micropyJes pOint toward the sides of the locule, but that the proximal ovule (lower.
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