Pollen Wall Development of Xiphidium coeruleum (Haemodoraceae) and its Systematic Implications

The development of the pollen grain wall in Xiphidium coeruleum(Haemodoraceae) was studied using TEM and cytochemical stainingtechniques. Microsporocyte ontogeny initiates with the degradationof the cellulosic cell wall and subsequent deposition of a thickcallosic cell wall. Following callose deposition, successivemeiosis occurs, resulting in a tetragonal tetrad of microspores.during meiosis, the cell walls of the tapetum break down, releasingthe syncytial periplasmodium. Irregular non-sporopollenous globularbodies are deposited in this peripheral periplasmodium, whichis rich in ER, golgi bodies, vesicles, and characteristic starchplastids. Within the microspore cytoplasm, vesicles, golgi bodies,and plastids are plentiful during the early tetrad stage. Atthis time the plasma membrane of the microspore develops characteristicevaginations. An extracellular membrane, the ‘white line’,is secreted outside the microspore plasma membrane, followedby callose wall degradation. Bead-like deposits of exine orprimexine are deposited at points along the ‘white line’simultaneously on inner and outer surfaces and opposite theoriginal plasma membrane evaginations. The bead-like exine depositscontinue to grow during the release of the microspores and developinto laterally appressed, rod-shaped ektexinous elements havinga tangentially oriented commissure, the vestige of the original‘white line’. The mature intine is two-layered,the outer exintine containing radially oriented vesicular structures,which are apparently derived from plasma membrane extensions.Exine development in Xiphidium is similar to ‘nexine 1’development in Lilium and may have evolved from an ancestraltectate-columellate condition by the loss of the sexine. Walldevelopment in members of the Zingiberales is strikingly similarto that reported here for the Haemodoraceae—evidence ofa possible relationship between the two taxa. Xiphidium coeruleum, Haemodoraceae, pollen, tapetum, development, exine     

The First Stages of Pre-harvest 'Peel Pitting' Development in 'Encore' Mandarin. An Histological and Ultrastructural Study

‘Peel pitting’ in the flavedo (the coloured epicarp)of ‘Encore’ mandarin arises as chlorotic spots.These spots correspond to parenchymal flattening and collapseof a variable number of sub-epidermal cell layers, which increasein parallel with the epidermis, extending between apparentlyhealthy zones. The first signs of cellular damage are associatedwith internal membrane disorganization of the plastids. Greatvesiculation of the cytoplasm occurs, followed by degradationof cytoplasmic membranes and accumulation of abundant osmiophilicmaterial, yielding amorphous and dense masses. The disorderstarted by September. Spotting was only observed on the outerside of the fruit and on portions of the rind most exposed toexternal factors. Radiation and high temperature of the rindover long periods may induce localized dehydration, plasmolysisand cell collapse.Copyright 1999 Annals of Botany Company Citrus, ‘Encore’ mandarin, pre-harvest pitting, peel disorder, histology, ultrastructure.     

Studies on Mature Seeds of Cuscuta pedicellata and C. campestris by Electron Microscopy

The seeds of Cuscuta pedicellata have been investigated by transmissionand scanning electron microscopy. Additional observations havebeen made on seeds of C. campestris by SEM only. The seed coatconsists of an outer single epidermis, two different palisadelayers, and an inner multiparenchyma layer. The outer epidermalwall in C. pedicellata has a thick cuticle and zones rich inpectic substances. The thicker ‘U-shaped’ cell wallsin the outer palisade layer are strengthened by a wall layerof hemicellulose. The inner palisade layer has thick walledcells with a ‘light line’. The inner cell wall ofthe compressed multiparenchyma layer has a thin cuticle. A fairlythick cuticle is positioned directly on the endosperm surface.The aleurone cell walls are different from the remaining endospermwalls. The latter are thick and believed to be of galactomannans.There is a ‘clear’ zone between the plasmalemmaand the cell wall in the aleurone cells. The embryo cells arepacked with lipids and proteins. In Cuscuta campestris mostendosperm has been absorbed during the seed development. Theembryo apex has two minute leaf primordia. The features of theCuscuta seeds are discussed in relation to functional and environmentalconditions. Cuscuta pedicellata, Cuscuta campestris, seed, seed coat, cuticle, cell walls, endosperm, aleurone cells, galactomannan, embryo, TEM, SEM     

Structure and Development of Plastids in Epidermal Cells of African Violet (Saintpaulia ionantha Wendl.) in Culture

The structure of plastids in epidermal cells of African violet(Saintpaulia ionantha Wendl.) ‘Marge Winters’ wasexamined using transmission electron microscopy before and afterplacement of leaf tissue in culture. The plastids from matureepidermal cells contained membrane-bound inclusion bodies andprolamellar bodies in various stages of development. Starchgrains and poorly developed granal stacks were observed in asmall number of plastids. After placement of the leaves on suitableculture medium, inclusion bodies decreased in size and the lamellarsystem became more organized. The plastids in the epidermaltissue are believed to be in an arrested state of developmentand are released from this state by placement on a medium inductiveto organogenesis. Saintpaulia ionantha Wendl., African violet, membrane-bound inclusion, tissue culture, plastid development     

Developmental rates of the copepods Calanus australisand Calanoides carinatus in the laboratory, with discussion of methods used for calculation of development time

The copepods Calanus australis and Calanoides carinatus developedfrom egg to adult in 20.3 and 18.3 days respectively at 15.5?Cand 16.0 and 12.0 days at 19.5?C. For both species the durationof the first two naupliar stages was short, <1 day. The thirdnaupliar stage was prolonged in C.australis but short in C.carinatus. Isochronal development was approximated from fourthnauplius through third copepodite; fourth and fifth copepoditestages were relatively long. Only female C.australis were produced;60% of the C.carinatus were female. Equiproportional developmentwas found for C.carinatus but not for C.australis. For bothspecies the variance in stage duration was established at thethird nauplius then remained unchanged through the older stages.In calculating developmental rates several commonly used methodswere compared. The method of ‘first appearance of an individualof a given stage’ and ‘mean time of appearance ofa stage’ both yielded estimates of development time thatwere 10–15% faster than the more commonly used methodof ‘median development time’. We suggest that allfuture work on developmental rates of copepods should use astandardized method for the calculation of stage-specific developmentalrates and that ‘median development time’ be themethod of choice. ³Present address: Marine Sciences Research Center, SUNY, StonyBrook, NY 11794-5000, USA     

Novel 'homeotic' CMS patterns generated in Nicotiana via cybridization with Hyoscyamus and Scopolia

Cytoplasmic hybrids (cybrids) that contain nuclear genetic materialfrom Nicotiana tabacum and cytoplasms from Hyoscyamus nigeror Scopolia carniolica were constructed by protoplast fusions.Both types of hybrids exhibited cytoplasmic male sterility (CMS).Furthermore, unusual floral morphogenesis marked by ‘greenflowers’ deprived of corolla and stamens occurred in themajority of the lines. Backcrosses of these plants with wild-typetobacco demonstrated a maternal inheritance of the ‘greenflower’ trait. After repeated transfer of cytoplasm (‘donor-recipientfusion’) from cytoplasmic hybrid N. tabacum (+H. niger)to albino plastome mutant N. tabacum DSR A15, male sterile tobaccoplants with two types of flowers were recovered (‘greenflowers’ and corolla-containing flowers with transformedstamens). RFLP analysis confirmed that N. tabacum (+ H. niger)and N. tabacum (+S. carniollca) as well as their sexual progeniescontained plastids from H. niger and S. carniolica, respectively.Mitochondrial DNA within the hybrids N. tabacum (+H. niger)originated from H. niger, but was obviously altered. Repeatedparasexual transmission, cybrids in the combination of N. tabacum+N.tabacum (+H. niger), reflected similar characteristics. Cybrids,N. tabacum (+S. carniolica) and their sexual progeny, whichresulted after pollination with wild-type tobacco, containeda modified mtDNA generally originating from tobacco. Furtherhistological analysis established the dramatic difference inthe composition of ‘green flowers’ and flowers ofwild-type tobacco. Therefore, the construction of tobacco cybridswith foreign cytoplasms provides a functional method for thede nova generation of alternative CMS types. Key words: Nicotiana, Hyoscyamus, Scopolia, cybrids, CMS, homeotic patterns     

Scanning Electron Microscopy of Silica Deposits in the Culms, Floral Bracts and Awns of Barley (Hordeum sativum Jess.)

Deposits of silica in the culm internodes, floral bracts andawns of Hordeum sativum Jess (cv Deba Abed) have been investigatedusing the scanning electron microscope The deposits were isolatedfrom all organic matter by digestion with nitric and perchloricacids Two basic types of deposits were recognized, lumen andwall silicification, the latter with or without lumen infillings In the culm internodes, lumen deposits are derived from idioblasts(‘hats’), sclerenchyma and xylem vessels In thefloral bracts they are derived from idioblasts (‘hats’and astenform opals), epidermal long cells (dendriform opals),sclerenchyma and xylem vessels The shape of these deposits doesnot generally resemble the outline of the cell itself, but depositsderived from cell walls do closely resemble the infact cell.In the culm, the walls of stomatal cells, trichomes and, largelythe outer tangential walls of mature long epidermal cells, becomesilicified, together with some cork cells In the floral bracts,silica is found in most epidermal cell walls but is confinedto the trichomes, scutiform cells and costal epidermal cellsearly in their development At maturity concentrations of silicaare much higher in the floral bracts and awns than in the culmsand leaves The results are discussed in relation to the pattern of depositionand its possible functions. Hordeum sativum Jess, barley, silica deposits, opals, scanning electron microscopy     

Structural Characteristics and Fatty Acid Composition of Psophocarpus tetragonolobus Seeds

Winged bean (Psophocarpus tetragonolbtis) seed coat in the hilarregion consists of a double layer of sclereids, tracheid barand spongy parenchyma cells. This is contrasted to the seedcoat structure on either side of the hilar region, which hasa single layer of sclereids, columnar cells and crushed parenchymacells. Cotyledonary cells are large (50 to 100µm in diameter)and have cell walls 2·4 to 4·7 µm thickwith pit-pair structures. Protein bodies and lipid bodies arethe main structural components of the cytoplasm while only asmall number of starch granules are present in each cell. Themajor portion of the lipid can be removed by non-polar solventsand contains oleic and linoleic acids as the predominant unsaturatedfatty acids. High levels of behenic acid were present in both‘free’ and ‘bound’ lipids. Psophocarpus tetragonolobus (L.) DC., winged bean, seed structure, seed coat, protein bodies, lipid bodies, fatty acids, oleic acid, linoleic acid     

Effects of the Introduction of Agrobacterium tumefaciens T-DNA ipt Gene in Nicotiana tabacum L. cv. Petit Havana SR1 Plant Cells

Transformation of tobacco leaf discs with the ‘cytokinin’ipt gene yielded several transgenic callus tissue lines, respectiveto the kind of ipt construction present in the A. tumefacienscointegrates. Those calli containing an active ipt gene wereable to grow hormone-autotrophically and showed an increasedendogenous cytokinin level in comparison with controls. Analysisof endogenous IAA level did not allow any quantitative correlationwith the cytokinin content. However, a minimal level of auxinseems to be necessary to obtain hormone-autotrophic growth.Exogenously supplied NAA significantly reduced the endogenouscytokinin content without modifying growth characteristics. The varying chlorophyll content in the different callus lineselicited the study of the ultrastructure of the plastids. Thecontrols contained small plastids, often filled with starchor accumulated vesicles that did not allow observation of theinternal membrane system. The ‘Pssu-ipt’ line, havinga higher cytokinin content, showed plastids with an internalmembrane system consisting of stroma and grana thylakoids, butthis structure was lost during subculture. Swollen thylakoidsappeared, the amount of starch was reduced and vesicles wereaccumulating. (Received November 15, 1990; Accepted March 4, 1991)     

Alteration of the Cell Surface during the Vegetative Cell Cycle of the Unicellular Green Alga Chlamydomonas reinhardtii

Alterations of the cell surface during the vegetative cell cycleof the unicellular green alga Chlamydomonas reinhardtii wereinvestigated using polyclonal antibodies against the purifiedand subsequently deglycosylated insoluble cell wall componentand against a 100 kDa polypeptide of the deglycosylated, chaotrope-solublewall fraction, respectively. Both antibodies recognized epitopeswithin the non-glycosylated domains of a ‘150 kDa’chaotrope-soluble glycoprotein (=GP3B) localized in the outerlayers of the C. reinhardtii cell wall. Immunofluorescence studiesindicated that both antibodies reacted with the surface of ‘late’sporangia (harvested 1 h before liberation of the zoospores),but not with the cell surfaces of released zoospores, growingcells and young sporangia, respectively. After pretreatmentwith aqueous LiCl, however, the cell surfaces of zoospores,growing cells and young sporangia became accessible to theseparticular antibodies. Highly purified preparations of the insolublewall fraction revealed strong immunofluorescence with both antibodiesbut not with the corresponding preimmune sera. Based on thesedata, we concluded that the antigenic sites of the insolubleglycoprotein framework of the C. reinhardtii wall are maskedby LiCl-soluble glycoproteins in single cell stages and youngsporangia, but not or to a lesser extent in the case of themother walls of ‘late’ sporangia. The conclusionwas supported by findings that (I) the multilayered structureof the mother-cell wall was disturbed in ‘late’,but not in young sporangia and that (II) the amounts of chaotropesolublecell wall glycoproteins present in the LiCl-extracts from intactsporangia decreased during ripening of the sporangia. (Received January 10, 1996; Accepted May 27, 1996)     

The Nitrogen Content of Plants and the Self-thinning Rule of Plant Ecology: A Test of the Core-skin Hypothesis

The ‘core-skin’ hypothesis postulates that secondarilythickened plants behave energetically as an inert ‘core’covered by an active ‘skin’, the ‘skin’being two-imensional, the ‘core’ three-dimensional.This would explain the ‘self-thinning ‘or‘–3/2’ rule of plant ecology, that is, the tendencyfor log (dry weight per plant) and log (number of plants perunit area) to progress along a straight line relationship, withslope = – 3/2’. The hypothesis was tested as follows. Plant nitrogen contentwas used as an estimate of the mass of ‘skin’ perplant, and dry weight as an estimate of the mass of the ‘core’.As plants mature the slope of the relationship between y = log(mass of nitrogen per plant) and x = log (mass of dry matterper plant) is expected to decline from an initial value of 1.0towards a final value of 0.66. The intercept of the relationshipis expected to reflect the intrinsic content of ‘skin’per unit of ‘core’. Genotypic variation in thisparameter should cause genotypic differences in the maximumattainable yield of biomass per unit area. The expectations were investigated by fitting the function y= p+qx+r exp – x to 30 sets of data on plant nitrogencontent, plant weight and time in 18 different vegetables. Simplelinear regressions of y on x were fitted to more limited setsof data on weights and nitrogen contents of mature trees. Theexpectations were, with some minor exceptions, confirmed. Nitrogen, yield, plant competition, self-thinning     

The growing outer epidermal wall: design and physiological role of a composite structure

Background: The cells of growing plant organs secrete an extracellular fibrouscomposite (the primary wall) that allows the turgid protoplaststo expand irreversibly via wall-yielding events, which are regulatedby processes within the cytoplasm. The role of the epidermisin the control of stem elongation is described with specialreference to the outer epidermal wall (OEW), which forms a ‘tensileskin’. Novel Facts: The OEW is much thicker and less extensible than the walls ofthe inner tissues. Moreover, in the OEW the amount of celluloseper unit wall mass is considerably greater than in the innertissues. Ultrastructural studies have shown that the expandingOEW is composed of a highly ordered internal and a diffuse outerhalf, with helicoidally organized cellulose microfibrils inthe inner (load-bearing) region of this tension-stressed organwall. The structural and mechanical backbone of the wall consistsof helicoids, i.e. layers of parallel, inextensible cellulosemicrofibrils. These ‘plywood laminates’ containcrystalline ‘cables’ orientated in all directionswith respect to the axis of elongation (isotropic material).Cessation of cell elongation is accompanied by a loss of order,i.e. the OEW is a dynamic structure. Helicoidally arranged extracellularpolymers have also been found in certain bacteria, algae, fungiand animals. In the insect cuticle crystalline cutin nanofibrilsform characteristic ‘OEW-like’ herringbone patterns. Conclusions: Theoretical considerations, in vitro studies and computer simulationssuggest that extracellular biological helicoids form by directedself-assembly of the crystalline biopolymers. This spontaneousgeneration of complex design ‘without an intelligent designer’evolved independently in the protective ‘skin’ ofplants, animals and many other organisms.     

Developmental control of xylem hydraulic resistances and vulnerability to embolism in Fraxinus excelsior L.: impacts on water relations

The freezing tolerance of many plants, such as pea (Pisum sativum),is increased by exposure to low temperature or abscisic acidtreatment, although the physiological basis of this phenomenonis poorly understood. The freezing tolerance of pea shoot tips,root tips, and epicotyl tissue was tested after cold acclimationat 2C, dehydration/rehydration, applications of 10^–4M abscisic acid (ABA), and deacclimation at 25C. Tests wereconducted using the cultivar ‘Alaska’, an ABA-deficientmutant ‘wil’, and its ‘wildtype’. Freezinginjury was determined graphically as the temperature that caused50% injury (T₅₀) from electrical conductivity. Endogenous ABAwas measured using an indirect enzyme-linked immunosorbant assay,and novel proteins were detected using 2-dimensional polyacrylamidegel electrophoresis. The maximum decrease in T₅₀ for root tissuewas 1C for all genotypes, regardless of treatment. For ‘Alaska’shoot tips and epicotyl tissue, exogenous ABA increased thefreezing tolerance by –1.5 to –4.0C, while coldtreatment increased the freezing tolerance by –7.5 to–14.8C. Cold treatment increased the freezing toleranceof shoot tips by –9 and –15C for ‘wil’and ‘wild-type’, respectively. Cold acclimationincreased endogenous ABA concentrations in ‘Alaska’shoot tips and epicotyls 3- to 4-fold. Immunogold labeling increasednoticeably in the nucleus and cytoplasm of the epicotyl after7 d at 2C and was greatest after 30 d at the time of maximumfreezing tolerance and soluble ABA concentration. Cold treatmentinduced the production of seven, three, and two proteins inshoot, epicotyl, and root tissue of ‘Alaska’, respectively.In ‘Alaska’ shoot tissue, five out of seven novelproteins accumulated in response to both ABA and cold treatment.However, only a 24 kDa protein was produced in ‘wil’and ‘wild-type’ shoot and epicotyl tissues aftercold treatment. Abscisic acid and cold treatment additivelyincreased the freezing tolerance of pea epicotyl and shoot tissuesthrough apparently independent mechanisms that both resultedin the production of a 24 kDa protein. Key words: Pisum sativum, cold acclimation, immuno-localization     

Freezing tolerance, protein composition, and abscisic acid localization and content of pea epicotyl, shoot, and root tissue 3

The freezing tolerance of many plants, such as pea (Pisum sativum),is increased by exposure to low temperature or abscisic acidtreatment, although the physiological basis of this phenomenonis poorly understood. The freezing tolerance of pea shoot tips,root tips, and epicotyl tissue was tested after cold acclimationat 2C, dehydration/rehydration, applications of 10^–4M abscisic acid (ABA), and deacclimation at 25C. Tests wereconducted using the cultivar ‘Alaska’, an ABA-deficientmutant ‘wil’, and its ‘wildtype’. Freezinginjury was determined graphically as the temperature that caused50% injury (T₅₀) from electrical conductivity. Endogenous ABAwas measured using an indirect enzyme-linked immunosorbant assay,and novel proteins were detected using 2-dimensional polyacrylamidegel electrophoresis. The maximum decrease in T₅₀ for root tissuewas 1C for all genotypes, regardless of treatment. For ‘Alaska’shoot tips and epicotyl tissue, exogenous ABA increased thefreezing tolerance by –1.5 to –4.0C, while coldtreatment increased the freezing tolerance by –7.5 to–14.8C. Cold treatment increased the freezing toleranceof shoot tips by –9 and –15C for ‘wil’and ‘wild-type’, respectively. Cold acclimationincreased endogenous ABA concentrations in ‘Alaska’shoot tips and epicotyls 3- to 4-fold. Immunogold labeling increasednoticeably in the nucleus and cytoplasm of the epicotyl after7 d at 2C and was greatest after 30 d at the time of maximumfreezing tolerance and soluble ABA concentration. Cold treatmentinduced the production of seven, three, and two proteins inshoot, epicotyl, and root tissue of ‘Alaska’, respectively.In ‘Alaska’ shoot tissue, five out of seven novelproteins accumulated in response to both ABA and cold treatment.However, only a 24 kDa protein was produced in ‘wil’and ‘wild-type’ shoot and epicotyl tissues aftercold treatment. Abscisic acid and cold treatment additivelyincreased the freezing tolerance of pea epicotyl and shoot tissuesthrough apparently independent mechanisms that both resultedin the production of a 24 kDa protein. Key words: Pisum sativum, cold acclimation, immuno-localization     

Observations on the Fine Structure of Plant Cell Walls II. The Microfibrillar Framework of the Parenchymatous Cell Wall in Cucurbita

The microfibrillar framework of parenchymatous walls in Cucurbitawas observed in petioles treated so as to remove various non-cellulosiccell wall components. Such extraction typically results in separationof the microfibrillar components into concentric lamellae. Thenumber and thickness of these lamellae vary according to theage and type of cell wall. The microfibrils appear to be orientatedwithin the plane of their lamellae but the orientation may varyin successive lamellae, and in many walls the crossed polylamellatecondition was detected. The collenchyma—and the outerepidermal cell walls show an alternation of lamellae with almostvertical microfibrils with those with a practically transverseorientation. In ordinary parenchymatous walls the alternationis not so extreme and is revealed only by the occasional presenceof the ‘herring bone pattern’ in non-radial sections.As a rule the lamellae are continuous around the circumferenceof a cell though individual lamellae may vary in thickness andsometimes appear to ‘fade out’. The present observationssuggest that growth of the primary wall occurs by depositionof microfibrils in successive lamellae thus confirming the basicpremise of the multinet theory of growth.     

Pyrimidine Nucleotide Biosynthesis in Vinca rosea Cells: Changes in the Activity of the de novo and Salvage Pathways during Growth in a Suspension Culture

Marked changes in the activity of the ‘de novo’and ‘salvage’ pathways of pyrimidine biosynthesisduring growth of Vinca rosea cells in a batch suspension culturewere observed. The activity of these pathways was investigated by determiningthe contribution of ¹⁴C of [2-¹⁴Cluracil, 12-¹⁴Cluridine. and[6-¹⁴Clorotate to the cell constituents and by measuring theactivity of the several enzymes of these pathways. During the lag phase of the culture, ‘uracil-’ and‘uridine-salvage’ pathways made the predominantcontribution to nucleotide biosynthesis, but, following theinitiation of cell division, the ‘de novo’ pathwayfor nucleotide biosynthesis operated appreciably. These results suggest that nucleotide synthesis during cellgrowth in a suspension culture can be divided into two stages:a ‘turnover stage’, during the lag phase of cellgrowth, and a ‘true biosynthetic stage’, which isinitiated in the cell division phase.     

The Effects of Salinity upon Galactosyl-Glycerol Content and Concentration of the Marine Red Alga Porphyra purpurea (Roth) C.Ag.

Changes in the major alcohol-soluble, low molecular weight carbohydratesof P. purpurea, O--D-galactopyranosyl-(1-2)-glycerol (‘floridoside’)and O--D-galactopyranosyl-(1-1)-glycerol (‘isofloridoside’),have been examined in response to salinity variation. ‘Floridoside’is shown to vary in absolute amount, increasing in hypersalineand decreasing in hyposaline media. ‘Isofloridoside’content per cell does not change in a similar manner. Responsesare almost identical under light or dark conditions, ‘floridoside’changes being complete within 24 h. Decreasing the externalwater potential using ionic and non-ionic solutes has the sameeffect upon galactosyl-glycerol content. The amount of ‘floridoside’synthesized, and degraded under hypersaline and hyposaline conditionsrespectively is shown to be insufficient to restore cell volumeto its original value. It is therefore suggested that the primaryfunction of ‘floridoside’ increases in concentratedsea-waters is that of a compatible solute, serving to protectthe cell during periods when the external salt content is increaseddramatically.     

Observations with the Electron Microscope on the Cell Structure of the Antheridium and Spermatozoid of Sphagnum

Sectioned antheridia of Sphagnum have been investigated withthe electron microscope at magnifications up to 50, 000 diametersand anatomical features of the main protoplasmic organs in twotypes of cells are described and illustrated. In the parenchymatouscells of the antheridial wall the main components encounteredare: nucleus, starch-laden plastids, mitochondria, fat bodies,tubules, putative golgi areas, vesicles, membranes, and granules.In the coiled spermatozoid the main observations concern thecoiled nucleus, the vestigial plastid, putative mitochondriaat the two extremities of the cell, the distribution of vesiclesand fibres in the cytoplasm, and the structure and mode of attachmentof the two flagella. In a comparative discussion it is suggestedthat the parenchymatous cells show resemblance to cells of thehigher plants, notably in details of their plastids and mitochondria.The extreme specialization of the spermatozoid impedes closecomparisons with other plants but a homology is tentativelysuggested between a peculiar organ referred to here as the ‘fibrousband’ to which the nucleus and both flagella are attachedand certain intercalary fibres originating in a comparable positionbetween the bases of the flagella in green and brown algae.     

Junction Complexes between Sieve Tubes in the Secondary Phloem of Myrtaceae

Junction complexes of unusual structure form between neighbouringsieve tubes in the secondary phloem of Eucalyptus species. Thick-walledribs support thin-walled ‘sieve areas’. In longitudinalsections the structures have a ‘concertina’- likeappearance. They are relatively large, up to 0.2 mm in length.Electron micrographs confirmed that the structures consistedof thin-walled areas perforated with pores, supported by muchthicker ribs. The structures provide a vast surface area fortransfer of metabolites between sieve tubes compared with thatof lateral wall sieve areas of other plants. Hydrolysis of parenchymacell walls occurs during the development of the junction complexes.The structures are only found when sieve tubes are in closeproximity and it is the redifferentiation and partitioning ofintervening parenchyma cells which result in junction complexformation. A survey for the presence of the structures in thephloem of other genera in the family Myrtaceae was made andthey were found in Tristania and Angophora but were not observedin Acmena and Metrosideros. Eucalyptus, sieve tubes, lateral walls, ultrastructure     

The Ultrastructure and Analytical Microscopy of Silicon Deposits in the Roots of Saccharum officinarum (L.)

Silicon accumulation in the endodermis of the ‘set’and ‘shoot’ roots of Saccharum officinarum (L.)were investigated by scanning electron microscopy and electron-probemicroanalysis. Silicon microassay was also carried out by meansof the Corinth analytical microscope (CORA). Aggregates arelargely associated with the inner tangential wall (ITW) of theendodermis and their formation is basically similar to thoseseen in Sorghum bicolor (L.) Moench. In contrast to Sorghumthe earliest deposits in Saccharum appear in wall strata wellwithin the cell wall cytoplasm interface. An additional layerof silicon was also located along the endodermal pericycle boundaryextending some distance along the middle lamella of the radialwalls. The results are discussed in relation to those of previous studiesof silicon accumulation in endodermal cells and to possiblefactors affecting such accumulations.