Stigmatic and stylar incompatibility in Prunus avium L.: Morphological and ultrastructural aspects

Abstract

Stigmatic and stylar structures of-sweet cherry (Prunus avium L.) «Malizia» were examined with light and electron microscopes (S.E.M. and T.E.M.).

It was demonstrated that the transmitting tissue, situated in the central portion of the style, extends below the stigmatic papillae and secretes an electrondense material accumulating in the intercellular spaces. Pollen tubes which germinated on the stigma, reached the ovules passing through this substance which facilitates their passage while trophic relations are established.

Some ultrastructural aspects of the pollen tubes observed inside the style were examined and discussed in relation to the phenomenon of self-incompatibility.     

Structural and Cytochemical Characteristics of the Stigma and Style in Vitis vinifera L. var. Sangiovese (Vitaceae)

Studies were carried out on structural and cytochemical aspectsof the stigma and style ofVitis vinifera . The stigma is ofthe wet papillate type with a continuous cuticle and pellicle.During the development of the papillae, the cell walls increasein thickness and produce a secretion product constituted oflipids that pass through the wall forming the exudate. The styleis solid with a central core of transmitting tissue which hasconspicuous intercellular spaces that increase remarkably fromthe periphery to the centre where the cuticle is present. Theintercellular spaces, where the pollen tubes grow, contain amatrix that includes polysaccharides, pectic substances andscattered areas of lipidic nature. Cytochemistry; stigma; style; ultrastructure; Vitis vinifera     

The Structure and Cytochemistry of the Pistil of Sternbergia lutea (Amaryllidaceae)

Studies were carried out on structural and cytochemical aspectsof the pistil of Sternbergia lutea (L.) KerGawl. The stigmais of the wet papillate type; the papillae are unicellular andare arranged densely around the rim of a funnel-shaped stigma.The stigma exudate is limited and is confined to the bases ofthe papillae and the inner lining of the stigma. The papillaeare smooth in the distal part and are covered with intact cuticle-pelliclelining. The cuticle is disrupted at places towards the baseof the papillae releasing the exudate. The exudate is rich inpectins and other polysaccharides but poor in proteins and lipids.The papillae show dense cytoplasmic profiles with extensiveendoplasmic reticulum (ER), abundant mitochondria, polyribosomesand active dictyosomes. The style is hollow. The stylar cavityis surrounded by two to four layers of glandular cells. In theyoung pistil the canal is lined with a continuous cuticle, butin the mature pistil the cuticle becomes disrupted and the canalis filled with the secretion produced by the cells of the surroundingglandular tissue. Ultrastructurally, the cells of the glandulartissue are very similar to the stigmatic papillae. The innertangential wall of the cells bordering the canal is uniformlythicker than other walls. The secretion in the stylar canal,as well as the intercellular spaces of the glandular tissue,stain intensely for pectins and polysaccharides but poorly forproteins and lipids. Pollen tubes grow through the stylar canal.Structural and cytochemical details of the pistil of Sternbergiaare compared with other hollow-styled systems. Pistil, Sternbergia lutea (L.) Ker-Gawl., stigma and style, structure and cytochemistry     

Structure and Cytochemistry of the Stigma and Pollen--Pistil Interaction in Zephyranthes

The stigma in Zephyranthes candida and Z. citrina is of thedry type with a continuous cuticle—pellicle. In some papillae,however, the terminal portion of the cuticle—pellicleis lifted upwards and occasionally even disrupted by the accumulationof a secretion product below it. Both non-specific esterasesand acid phosphatases are present on the stigma surface. Thestyle is solid with a central core of transmitting tissue whichhas conspicuous intercellular spaces containing a matrix thatincludes proteins, polysaccharides and pectic substances. Zephyranthes citrina is self-compatible while Z. candida isself-incompatible. Followng incompatible pollination in Z. candida,pollen germination is normal but pollen tube growth is inhibitedat the junction of the stigma and style. Self-incompatibilitycan be overcome by bud pollination. Protein synthesis is necessaryfor pollen germination in both species. Concanavalin A bindsto the stigma surface of both species, but does not affect pollentube penetration in Z. candida. In crosses between the two speciestypical unilateral incompatibility is observed when Z. candidais used as the pistillate parent. Zephyranthes, stigma-surface enzymes, dry stigma, pollen-pistil interaction, self-incompatibility, unilateral-incompatibility     

Pollen-Stigma Interaction in the Leguminosae: the Organization of the Stigma in Trifolium pratense L.

The intact stigma of Trifolium pratense possesses a smooth receptivesurface fringed by a few ranks of brush hairs. This surfaceis ensheathed by a thin (75–100 nm) but highly impermeablecuticle, which encloses four to five ranks of secretory cellsimmersed in their secretory products. Experimental single-grainpollinations show that pollen cannot become hydrated or germinateon the intact surface. The cuticle is ruptured when the floweris tripped; the secretion is released, and captured pollen-selfor cross-can then germinate. As in other papilionoid Leguminosae,this mechanism provides a guard against premature selling. Thesecretory cells are elongated; they remain in communicationthrough persistent pit-fields as the intercellular spaces fillwith secretion product. The secretion forms a lipid-rich emulsion,with a mucilaginous aqueous phase which reacts cytochemicallyfor protein and carbohydrate and has esterase activity. Duringthe early development of the stigma head, the cells possessa fine-structure appropriate to their secretory function, withabundant ribosomal and smooth endoplasmic reticulum, stratifiedor in the form of ramifying and anastomosing tubules, numerousmitochondria and a well developed Golgi system. Lipid globuli,partly invested in endoplasmic reticulum, are abundant in theyoung cells, but there is as yet no indication of how the lipidis transferred to the intercellular spaces during the secretoryperiod. As the stigma matures, the secretory cells become moribund. Leguminosae, Trifolium pratenseL., pollen-stigma interaction, self-incompatibility, autofertility, stigma secretory system, lipid secretion, cuticle permeability     

Pollen--stigma Interaction in the Leguminosae: the Secretory System of the Style in Trifolium pratense L.

The canal that traverses the upper part of the style of Trifoliumpratense is derived lysigenously. The core tissue of the veryyoung style consists of elongated cells similar to those ofthe transmitting tissue of solid-style families such as theSolanaceae; as the style matures, these cells separate to formthe canal, which receives secretions both from the core tissueand the inner wall cells. The early secretion of proteins intothe intercellular spaces is associated with the presence ofparamural bodies (lomasomes) in the adjacent cells. In the cellsin the immediate vicinity of the canal, vesicles, probably derivedfrom the Golgi system enlarge during later development and accumulatea protein-carbohydrate content, which is later passed into thecytoplasm where it forms densely packed fibrillar nodules. Withthe dissolution of the cell membranes, this material is passedinto the canal, where it is progressively diluted by continuedingress of water until the cavity reaches its final volume. Leguminosae, Trifolium pratense L., pollen—stigma interaction, self-incompatibility, stylar secretion, protein secretion     

The Structure of Nitrogen Fixing Root Nodules on the Aquatic Mimosoid Legume Neptunia plena

Nodules of the aquatic mimosoid legume Neptunia plena (L.) Benth.were always found associated with roots but not stems. Theyappeared macroscopically 10 and 20 d after inoculation on plantsgrown hydroponically and in vermiculite, respectively. The developmentof nitrogen-fixing cells occurred in a series of stages notyet reported in legume nodule formation: initial infection wasapparently intercellular and rhizobia spread between cells andthrough intercellular spaces before penetrating individual hostcells by means of infection threads. Subsequently nodule developmentwas broadly similar to that described for indeterminate papilionoidnodules. The infection threads of Neptunia and pea nodules containeda matrix with a common epitope, which was, in Neptunia, extrudedfrom the infection thread at the point of bacterial release. The central tissue contained infected and interstitial cellsand was surrounded by a three-layered cortex and a phellem.Bounding the infected region was a layer two to three cellsthick with large, unoccluded intercellular spaces. Externalto this was a layer, one or more cells thick, in which the cellwalls were interlocked, reducing the number of radially orientedintercellular spaces. The outer layer, several cells thick,contained intercellular spaces many of which were occluded.These features did not vary with growth conditions in a waywhich might influence oxygen diffusion characteristics. However,the phellem of water-cultured nodules was much more aerenchymatousthan that of vermiculite-grown nodules. Aquatic legume, Neptunia plena, nitrogen fixation, oxygen, root nodules, Rhizobium     

Pollen-pistil Interaction in a Non-pseudogamous Apomict,Commiphora wightii

Structural and cytochemical details of the pistil and the interactionof pollen and pistil were studied in a non-pseudogamous apomict,Commiphorawightii.The anthers in the male and bisexual flowers producefunctional pollen grains. The stigma is of the wet and papillatetype. The style is typically solid with two strands of transmittingtissue that traverse the entire length of the style. There isa marked reduction in the area occupied by the transmittingtissue from the stigma to the base of the style. The cells ofthe transmitting tissue are isodiametric in transverse as wellas longitudinal section and do not form longitudinal files ofelongated cells as reported for other taxa. Proteins could notbe localized in the intercellular matrix. Although pollen grainsgerminate on the stigma, pollen tubes do not grow beyond theproximal one third of the style. Changed orientation of thecells of the transmitting tissue and absence of proteins inthe intercellular matrix could account for the failure of thepistil to support pollen growth.Copyright 1998 Annals of BotanyCompany Guggul, pollen-pistil interaction, non-pseudogamous apomict,Commiphora wightii, transmitting tissue     

The Structure and Cytochemistry of the Stigma-style Complex ofCorylus avellanaL. 'Tonda Gentile delle Langhe' (Corylaceae)

Structural and cytochemical aspects of the stigma-style complexofCorylus avellanawere studied. In cross section the stigmaticstyle consists of papillae, one or two layers of sub-epidermalcells and a central transmitting tissue. The papillae coverthe style for about 80% of its length, are unicellular and arecoated with a cuticle-pellicle. During development, the cellwalls of the papillae increase in thickness and between them,below the cuticle, lipid bodies are observed. The sub-epidermalcells are similar in cell content to the papillae. The centraltransmitting tissue consists of highly vacuolated cells andthe intercellular spaces are filled with a proteic and polysaccharidicsubstance. Both the transverse and longitudinal walls containplasmodesmata.Copyright 1998 Annals of Botany Company cytochemistry, stigma and style, ultrastructure,Corylus avellana     

Development and Histochemistry of the Pistil of the Grape, Vitis vinifera

The development of the grape pistil is followed for a periodof 9 weeks from flower initiation to anthesis. Three phasesof pericarp differentiation are revealed: ring meristem formation;cell proliferation by anticlinal cell divisions; and a maturationphase characterized by periclinal cell division and differentiation.Both the stigma papillae and the transmitting tissue of thestyle originate by periclinal cell divisions. The receptivestigma is of the wet type and comprises many filamentous papillae,each composed of about 20 cells and covered by a loose cuticle.The stigma exudate shows similar cytochemical properties tothe material in the intercellular spaces of the transmittingtissue and is physically continuous with it. After pollinationand coincident with withering of the stigma, a single layerof stylar cells becomes suberized, forming a protective layerof cicatrix. Vitis vinifera, grape, pistil, development, histochemistry     

The Transmitting Tissue in Brugmansia suaveolens L.: Ultrastructure of the Stylar Transmitting Tissue

The stylar transmitting tissue of the angel's trumpet, Brugmansia(Datura) suaveolens, was studied at two developmental stages:about 6 d before anthesis and after anthesis. Histochemicallocalization of polysaccharides was carried out with PATAg andimmunohistochemistry with gold-conjugated antibodies recognizingpectins. Before anthesis the transmitting tissue forms a centralcore of polyhedral meristematic, still dividing, cells withnarrow intercellular spaces. Epitopes for unesterified pectinsare present in the walls and the spaces between the cells, whilemethylesterified pectins are confined to the middle lamellaand intercellular spaces. PATAg positive material and the antibodyagainst unesterified pectin was found in plasmalemma invaginationsand multivesicular bodies. Dictyosome cisternae and vesiclescontained epitopes for both kinds of pectins. Plastids are poorlydifferentiated and lack starch. Nutrients are stored as lipidbodies, which are digested by small vacuoles. After anthesisthe transmitting tract cells form cylindrical files separatedby voluminous spaces filled with a mucous secretion reactingwith PATAg and with the antibody against unesterified pectins.Dictyosome vesicles contain epitopes for the same kind of pectins.The cells are vacuolized and have leucoplasts. This study showspectin synthesis by different parts of the endomembrane systemand changes in pectin esterification during stylar development.Copyright1993, 1999 Academic Press Brugmansia suaveolens, immunocytochemistry, pectin, secretion, style, transmitting tissue     

Developmental Aspects of Ultrastructure, Histochemistry and Receptivity of the Stigma of Nicotiana sylvestris

The bilobed papillate stigma of Nicotiana sylvestris Speg. andComes, is covered at maturity with a copious exudate containinglipid, protein and carbohydrate. The stigma is receptive fromthe very early stage of development and it also stains positivelyfor esterase activity. The stigma has three distinct zones:an epidermis with papillae; a subepidermal secretory zone; anda parenchymatous ground tissue. The behaviour of the cells ofthese three zones has been followed from 6 d before anthesisto one day after anthesis and pollination. The cells of theepidermis and the secretory zone stain intensively for lipids,proteins and carbohydrates in the initial stages. The secretoryzone develops large intercellular spaces containing heterogenoussecretory products which also stain positively for the aforesaidthree compounds. At maturity the secretory products are releasedto the surface through gaps formed in the epidermis by cellseparation. The main secretion of the stigma is produced bythe cells of the secretory zone. Less secretion is derived fromthe stigmatic papillae. Some amount of secretion is also releasedfrom the stylar transmitting tissue adjoining the stigma. Theglandular cells of the stigma contain numerous plastids, mitochondria,ribosomes, ER, cytoplasmic lipid droplets and some dictyosomes.The plastids and the vacuoles in the secretory cells of thestigma have a lot of electron dense (osmiophilic) inclusionsrespectively in the initial and later stages of development.The former are probably involved in the production of thesematerials. It is suggested that the proteins are directly secretedby rough ER compartments whereas smooth ER is involved in thesynthesis of lipidic materials. The carbohydrate moiety of theexudate is released by the eccrine mode (sugar mono- and dimers)with some addition of polymers by disintegration of the middlelamellae. The means by which the lipidic and osmiophilic materialis extruded remains unclear. Nicotiana sylvestris, stigma receptivity, organization, stigmatic secretory system, stigmatic exudate     

Pollen-Stigma Interaction in the Leguminosae: Stigma Organization and the Breeding System in Vicia faba L.

As in other papilionoid Leguminosae, the receptive surface ofthe stigma of Vicia faba L. is invested by a detached cuticle.This cuticle, the so-called ‘stigma membrane’ ofplant breeding literature, is lifted away from the epidermisduring development by the accumulation of a lipid-rich secretionreleased into the intercellular spaces of the stigma head bythe epidermal cells and the underlying three to four cell layers.The cuticle is thickened over the prominences left by the epidermalpapillae, thinning out between. Pollen, whether self or cross,cannot hydrate and germinate in contact with the intact stigmasurface, but must await the disruption of the cuticle and therelease of the retained secretion. In most genotypes this takesplace only when the flower is tripped by visiting pollinatorsor in consequence of severe agitation by wind. A comparison of lines differing in their degree of autofertilityin field conditions has revealed various differences in stigmastructure. A highly autofertile line had low papillae at thereceptive tip, with a relatively thin intervening cuticle, whilean autosterile line had longer papillae and a thicker cuticle.A line with partial autofertility was intermediate in thesecharacteristics. These properties of the stigma surface, togetherwith other differences in flower structure, are probably adequateto account for the variation in the degree of autosterility,since no evidence was obtained suggesting the presence of aneffective physiological self-incompatibility system in any ofthe lines studied. Since the rupturing of the stigma cuticle is affected by theturgor pressure of the cells of the stigma and style, some degreeof environmental interaction is to be expected: autofertilityshould be at the highest in conditions of adequate water supply,and lowest where there is water deprivation. Pollen-stigma interaction, Vicia faba L., Leguminosae, breeding system, self-incompatibility     

Estimation of genetic diversity in some Iranian wild Prunus subgenus Cerasus accessions using inter-simple sequence repeat (ISSR) markers

As Iran is one of the main origins of Prunus germplasm. In this study, ISSR markers were used for genetic diversity evaluation of 39 accessions of subgenus Cerasus belonging to six species i.e. Prunus avium L., Prunus cerasus L., Prunus mahaleb L., Prunus incana Pall., Prunus microcarpa Boiss., and Prunus brachypetala Boiss.. With 12 ISSR primers, 151 polymorphic bands were detected with polymorphism ratio range of 81.8%–100%. The lowest similarity (0.04) was found between P. avium and P. microcarpa genotypes and the mean of similarity between all genotypes was 0.28. Cluster analysis separated improved cultivars from wild accessions. Improved cherry cultivars and rootstocks were placed closer to the P. avium than the other species. The principal coordinate analysis (PCoA) supported the cluster analysis results. The wild accessions were separated according to their species and collection sites. ISSR markers are useful techniques for genetic diversity evaluation in Prunus subgenus Cerasus.     

Phloem Mobility of Boron is Species Dependent: Evidence for Phloem Mobility in Sorbitol-rich Species

Boron is generally considered to be phloem immobile or to haveonly limited phloem mobility in higher plants. Evidence suggests,however, that B may be mobile in some species within thePyrus,Malus andPrunusgenera. These genera utilize sorbitol as a primarytranslocated photosynthate and it has been clearly demonstratedthat B forms stable complexes with sorbitolin vitro.In the researchpresented here we demonstrate, further, that B is freely phloemmobile inPyrus, MalusandPrunusspecies and suggest that thisis mediated by the formation and transport of B-sorbitol complexes. The pattern of B distribution within shoot organs and the translocationof foliar-applied, isotopically-enriched¹⁰B was studied in sixtree species. Results demonstrate that in species in which sorbitolis a major sugar (sorbitol-rich), B is freely mobile while inspecies that produce little or no sorbitol (sorbitol-poor) Bis largely immobile. The sorbitol-rich species used here werealmond [Prunus amygdalusB. syn.P. dulcis(Mill.)], apple (MalusdomesticaB.) and nectarine (Prunus persicaL. B. var.nectarinaM.),sorbitol-poor species included fig (Ficus caricaL.), pistachio(Pistacia veraL.) and walnut (Juglans regiaL.). In sorbitol-richspecies foliar applied¹⁰B was transported from the treated leavesto adjacent fruit and specifically to the fruit tissues (hull,shell or kernel) that developed during the experimental period.Whereas, foliar-applied¹⁰B was rapidly translocated out of leaves,only a small percentage of the¹¹B present in the leaf at thetime of foliar application was retranslocated. In sorbitol-richspecies, B concentrations differed only slightly between oldand young leaves while fruit tissue had significantly greaterB concentrations than leaves. In contrast, sorbitol-poor specieshad significantly higher B concentrations in older leaves thanyoung leaves while fruit tissue had the lowest B concentration.This occurred irrespective the source of plant B (soil, solutionor foliar-applied). In a subsequent experiment the growth ofapple trees in solutions free of applied B was maintained solelyby foliar applications of B to mature leaves. These resultsindicate that B is mobile in species that produce significantamounts of sorbitol. We propose that the mobility of B in thesespecies is mediated by the formation of B-sorbitol complexes. Almond; Prunus amygdalus ; apple; Malus domestica; nectarine; Prunus persica; fig; Ficus carica; pistachio; Pistacia vera; walnut; Juglans regia; boron; phloem mobility; deficiency; toxicity; inductively coupled plasma-mass; spectrometer     

Amitosis and Endocytogenesis in the Fruit of Malus sylvestris

Cyto-histological investigations on the initiation and developmentof a non-pathogenic physiological ‘cork spot’ necrosisin the outer cortex of the fruit of Malus sylvestris Mill. ‘YorkImperial’ revealed two distinct aberrancies, namely, amitoticnuclear division and intracellular or endogenous proliferationswithout hyperplasia per se The incipient necrotic conditionbecomes evident internally about 3 weeks after fruit set asminute isolated, discoloured, amorphous spots of disorganizedruptured cells within otherwise healthy cortical tissue Thedisorganization continues slowly with adjoining cells becomingsimilarly necrotic After the lysis of the initial cork spotcells, about 1 month later, sporadic cellular changes occurin various healthy vacuolated cortical cells contiguous to andencompassing the necrotic tissues. The nuclei increase in sizeand volume as they assume distinctive positions in preparationfor amitotic nuclear divisions. The enlarged nucleus or macronucleus,containing one or several nucleoli, divides by a distinct cleavagedeveloping from a constriction perpendicular to its longitudinalaxis The division results in the amitotic formation of two daughtermicronuclei that usually become separated by the formation ofa cell wall. No evidence of cell plate formation was observedand the method of cell wall formation could not be determined.Repeated amitotic divisions of the micronuclei result in anintracellular or endocytogenetic proliferation of parenchymatouscells that are invariably confined within the original mothercell until its wall ruptures The endogenous proliferations arereleased into lacunae or intercellular spaces, eventually becomedisorganized, and disintegrate, with the accumulated residualincrements resulting in an overall ‘cork spot’ appearance. Malus sylvestris Mill., apple, amitosis, endocytogenesis, multinucleate cells, macronuclei, micronuclei     

Seasonal Dimorphism in the Mediterranean Cistus incanus L. subsp. incanus

Mediterranean perennial species are described as being sclerophyllous,or summer deciduous, or seasonally dimorphic. Field observationin the coastal maquis of Castelvolturno Nature Reserve, southernItaly, showed thatCistus incanus L. subsp. incanus is a seasonallydimorphic species as it develops brachyblasts with small leavesin summer, and dolichoblasts with large leaves in winter. Fieldbiometric data confirmed that winter shoots were 14-times longerthan those developed in summer and had many more leaves. Thearea of single winter leaves was five-times that of summer leaves.Anatomical leaf structure also changed with the season: winterleaves were flat while summer leaves had a crimped lamina whichwas partially rolled to form crypts in the lower surface. Leaveswere covered by considerably more trichomes in summer than inwinter. Stomata were uniformly distributed along the lower epidermisof winter leaves but were only present in the crypts of summerleaves. In summer leaves, a palisade layer was often found onboth sides of the lamina, the mesophyll cells were generallysmaller and the intercellular spaces were reduced. Winter leaveshad a dorsiventral structure and larger intercellular spaces.Seasonal dimorphism is generally reported to be an adaptationto summer drought. However, the morphology and anatomy of C.incanus L. subsp.incanus showed that the subspecies has notonly developed a strategy to survive summer drought, but hasevolved two different habits, one more xerophytic than the other,to optimize adaptation to the seasonal climatic changes occurringin Mediterranean environments. Copyright 2001 Annals of BotanyCompany Cistus, Cistus incanus L. subsp. incanus, climatic changes, leaf anatomy, leaf dimorphism, Mediterranean shrubs, phenology, seasonal dimorphism     

Structural and Cytochemical Analysis of the Stigma and Style in Tibouchina semidecandra Cogn. (Melastomataceae)

Structural and cytochemical aspects of the pistil of Tibouchinasemidecandra Cogn. were studied. The stigma is of the wet-papillatetype and is structurally divisible into a papillar zone anda stigmatic zone. The papillar zone consists of loosely arrangedpapillae which are matchstick-shaped, unicellular, and producelipid droplets that remain entrapped below the thick cuticle.The bulk of cell volume is made up of large vacuoles rich intannin. The stigmatic zone consists of layers of secretory cellswith dense cytoplasm, actively secreting dictyosomes and numerousrough endoplasmic reticulum (RER) profiles. Free-flowing lipidexudate, produced by these cells, is initially stored in theintercellular spaces, and subsequently extruded out to coverthe surface. The style is solid with a core of transmittingtissue traversing its whole length. The transmitting tissueconsists of loosely arranged cells with numerous organellesand conspicuous intercellular substance rich in polysaccharidesand pectins. Ultrastructural details indicate that the intercellularsecretion is accompanied with fraying of the wall component.Both the transverse and longitudinal walls contain plasmodesmata.Copyright1995, 1999 Academic Press Cytochemistry, stigma and style, ultrastructure, Tibouchina semidecandra     

Abscisic Acid in Prunus Trees: Isolation and the Effect on Growth of Excised Shoot Tissue

The abscisic acid content of the mature shoot tissue of vigorous Prunus avium, medium sized Prunus cerasus and dwarfing Prunus cerasus self (S) was estimated by spectropolarimetry and gas chromatography. There is no correlation between the levels of ABA found in the tissue and the dwarfing potential of the trees investigated. However, Prunus arium stem tissue shows the lowest sensitivity to ABA, whereas the explants of normal and dwarfing Prunus cerasus exhibit a stronger inhibition.     

On the Occurrence of Intracellular Blue-green Algae in Cortical Cells of the Apogeotropic Roots of Macrozamia communis L. Johnson

The occurrence of species of the cyanophytes Nostoc and Anabaenain the cortex near the algal zone is reported for apogeotropicroots of Macrozamia communis L. Johnson. Algae were found tooccur both intercellularly and intracellularly in cells of theinner and outer cortex. This is the first record of intracellularalgae in the cycads and only the second report of this phenomenonin vascular plants. By examination of cells at various stagesof invasion by algae, it is interpreted that algal invasionof cortical cells and intercellular spaces is preceded by mucusapparently secreted by algal zone cells of the host, and depositedin intercellular spaces of cortical parenchyma cells nearby.Also algal penetration of cortical cells is preceded by an algalinvasion front of finely granular mucal material which bypassesmucus already deposited in intercellular spaces and may eitherlyse part of the host cell wall or enter through the plasmo-desmata,filling much of the cell cavity. Subsequently, large numbersof the algal symbionts enter the cell and may be enclosed withinhost wall material. Electron microscopic techniques are nowbeing employed to further clarify these invasion processes.