Another View of the Ultrastructure of Cucurbita Phloem

The primary phloem of young internodes of Cucurbita maxima wasstudied with the electron microscope. Phloem parenchyma cellsare highly vacuolated and contain nuclei, endoplasmic reticulum,ribosomes, mitochondria, chloro-plasts, and occasional dictyosomes.As compared with parenchyma cells, the most distinctive featuresof companion cells are their extremely dense cytoplasm, lowdegree of vacuolation, lack of chloroplasts, and numerous sieve-elementconnexions. Companion cells contain plastids with few internalmembranes. At maturity the enucleate sieve element is linedby a plasmalemma, one or more cistema-like layers of endoplasmicreticulum, and a membrane which apparently delimits the parietallayer of cytoplasm from a large central cavity. In OsO_4–-andglutaraldehyde-fixed elements, the central cavity is traversedby numerous strands, which run from cell to cell through thepores of sieve plates and lateral sieve areas, and which arederived ontogenetically from the slime bodies of immature cells.Numerous normal-appearing mitochondria are present in the parietallayer of cytoplasm. The pores of sieve plates and lateral sieveareas are lined with cytoplasm. The ultrastructural detailsof young sieve elements differ little from those of other youngnucleate cells. During sieve-element development, the sieveelement increases in vacuolation. At the same time, slime bodiesdevelop in the cytoplasm. With glutaraldehyde fixation, thesebodies often exhibit a double-layered limiting membrane. Asthe sieve element continues to differentiate, the slime bodiesincrease in size and the parietal layer of cytoplasm becomesvery narrow. Presently, the slime bodies begin to disperse andtheir contents fuse. This phenomenon occurs in the parietallayer of cytoplasm, while the latter is still delimited fromthe large central vacuole by a distinct tonoplast. The initiationof slime-body dispersal more or less coincides with perforationof the pore sites, and many pores are traversed by slime earlyin their development. Before slime-body dispersal, all dictyosomesand associated vesicles disappear from the cytoplasm. Eventually,the tonoplast diappears and the slime becomes distributed throughoutthe central cavity in the form of strands. Nuclei and ribosomesdisappear before breakdown of the tonoplast. Sieve elementsare connected with companion cells and parenchyma cells by plasmodesmata.     

Ultrastructural features of the salt gland of Tamarix aphylla L.

Summary The salt gland in Tamarix is a complex of eight cells composed of two inner, vacuolate, collecting cells and six outer, densely cytoplasmic, secretory cells. The secretory cells are completely enclosed by a cuticular layer except along part of the walls between the collecting cells and the inner secretory cell. This non-cuticularized wall region is termed the transfusion are (Ruhland, 1915) and numerous plasmodesmata connect the inner secretory cells with the collecting cells in this area. Plasmodesmata also connect the collecting cells with the adjacent mesophyll cells.There are numerous mitochondria in the secretory cells and in different glands they show wide variation in form. In some glands wall protuberances extend into the secretory cells forming a labyrinth-like structure; however, in other glands the protuberances are not extensively developed. Numerous small vacuoles are found in some glands and these generally are distributed around the periphery of the secretory cells in association with the wall protuberances. Further, an unusual structure or interfacial apparatus is located along the anticlinal walls of the inner secretory cells. The general structure of the gland including the cuticular encasement, connecting plasmodesmata, interfacial apparatus, and variations in mitochondria, vacuoles, and wall structures are discussed in relation to general glandular function.     

A highly differentiated glomeromycotean association with the mucilage-secreting, primitive antipodean liverwort Treubia (Treubiaceae): clues to the origins of mycorrhizas

Thallus anatomy in three species of the primitive liverwort genus Treubia (Metzgeriidae, Treubiales) was studied by light and electron microscopy. The thallus exudes copious mucilage, a feature shared elsewhere in liverworts only with the mycotrophic subterranean axes of the allied genus Haplomitrium. The central strand in the thallus midrib has a unique histological organization and harbors an intra- and intercellular infection by a glomeromycotean fungus that is far more highly differentiated than most of the glomeromycotean associations described to date. The fungus enters the thallus via clefts in the ventral epidermis along the midrib and colonizes the parenchyma above, forming intracellular coils and prominent, relatively short-lived, hyphal swellings. Above the zone with intracellular colonization is a tissue area containing mucilage-filled intercellular spaces; here the fungus is entirely intercellular and forms abundant pseudoparenchymatous structures and, in more mature parts of the thalli, large hyphae with thick multistratose walls. Mucilage in Treubia differs in histochemistry and origin from that produced by apical papillae, via hypertrophied Golgi, in all other bryophytes. Remarkable parallels between fungal associations in Treubia, Haplomitrium, and Lycopodium, all members of very ancient lineages, suggest that these associations epitomize very early stages in the evolution of glomeromycotean symbioses.     

ULTRASTRUCTURE OF THE SECONDARY PHLOEM OF TILIA AMERICANA

Summer and winter (July and January) samples of secondary phloem of Tilia americana were studied with the electron microscope. Parenchyma cells contain: nuclei, endoplasmic reticulum, ribosomes, plastids, mitochondria and occasional dictyosomes. Well-defined tonoplasts separate vacuoles from cytoplasmic ground substance. Vacuoles often contain tannins. Lipid droplets are common in cytoplasm. Endoplasmic reticulum–connected plasmodesmata are aggregated in primary pit fields. Companion cells differ from parenchyma cells in having numerous sieve-element connections, possibly slime, and in lacking plastids. Mature, enucleate sieve elements possess 1–4 extruded nucleoli. Numerous vesicles occupy a mostly parietal position in association with plasmalemma. The mature sieve element lacks endoplasmic reticulum, organelles (except for few mitochondria) and tonoplast. In OsO_4– and glutaraldehyde-fixed elements, slime has a fine, fibrillar appearance. Normally, these fine fibrils are organized into coarser ones which form strands that traverse the cell and the plasmalemma-lined pores of sieve plates and lateral sieve areas.     

SECONDARY PULVINUS OF ROBINIA PSEUDOACACIA (LEGUMINOSAE): STRUCTURAL AND ULTRASTRUCTURAL FEATURES

The structure of the secondary pulvinus of Robinia pseudoacacia has been examined together with ultrastructural features of motor cells both in open and closed pulvini, to identify ultrastructural changes associated with leaflet movement. Pulvini have a central vascular core bordered by thick-walled collenchyma cells, which in turn are surrounded by several layers of cortical parenchyma cells. Cortical motor cells exhibit ultrastructural features similar to those reported in homologous cells of other pulvini. The vacuolar compartment contains two kinds of vacuoles: nontannin vacuoles, which change both in number and size during leaflet movement, and tannin vacuoles, which may act as an ion reservoir. No differences in wall thickness were found between flexor and extensor motor cells. Thick walls of collenchyma cells show numerous pits with plasmodesmata through which the phloem parenchyma cells and the inner cortical motor cells are connected. Tannin vacuoles and calcium oxalate crystals are common inclusions of phloem parenchyma cells. The tissue arrangement and the occurrence of pits with plasmodesmata in the central cylinder cells provide evidence of symplastic continuity through the central cylinder between the extensor and flexor regions of the motor organs. The greater amplitude of Robinia leaflet movements may be related to the extension of motor regions, the scarcity of lignification in the central vascular core, and the thin flexor walls.     

Localization of ATPase in Sink Tissues of Ricinus

Histochemical localization of ATPase was carried out on phloemtissues from vegetative and reproductive sinks of Ricinus communis,using lead precipitation procedures. Reaction products werelocalized mainly at the plasma membrane of the sieve elements,companion cells and phloem parenchyma cells. Activity was alsopresent in plasmodesmata, the tonoplast of companion cells anddispersed P-protein within the sieve element lumen. The resultsare discussed in relation to the possible involvement of a plasmamembrane ATPase in apoplastic and symplastic unloading fromthe phloem conducting tissues. ATPase, sink tissues, unloading, Ricinus communis     

Microtuboli in Cellule Parenchimatiche ed Epidermiche di Coleottile di Avena

Abstract

Microtubules in parenchyma and epidermis cells of avena coleoptile. — The fine structure of differentiating parenchyma and epidermis cells of the oat coleoptile, fixed in glutaraldehyde-osmium tetroxide, or glutaraldehyde-potassium permanganate, was investigated. Tubular structures have been observed aligned in the peripheral cytoplasm, between the cell wall and the plasma membrane, embedded in the cell wall and inside the tonoplast in the vacuoles.

The nature and function of these structures are yet unknown. Microtubular structures, localized beneath and above the plasma membrane, have been associated to the wall development; the function of the microtubules observed in vacuoles results, anyhow, of far problematic interpretation.     

The Formation of Catenate Foliar Gemmae and the Origin of Oil Bodies in the Liverwort Odontoschisma denudatum (Mart.) Dum. (Jungermanniales): a Light and Electron Microscope Study

This article describes the ultrastructural events associatedwith the differentiation and liberation of the exogenous gemmaeproduced in branched acropetal chains along the margins of theleaves in the liverwort Odontoschisma denudatum. Formation ofa dorsal protrusion from young leaf cells containing a largecentral nucleus, small vacuoles, starch-free chloroplasts, scatteredcytoplasmic lipid droplets but no oil bodies, signals the onsetof the formation of the initial cell of a gemmiferous filament.The protrusion enlarges and the nucleus migrates into its base,therein dividing with the equator of the spindle virtually fillingthe central isthmus between the leaf surface and the now swollentip of the initial cell. Subsequent divisions of the initialcell produce a chain of cells in atropetal succession. Transverselyorientated microtubules line the cortical cytoplasm along thelateral walls of the terminal cells of the gemmiferous filaments,but are absent from the tips, thus suggesting that these cellselongate by intercalary, rather than by tip, growth. Duringmitosis microtubules are closely associated with the envelopesof spindle-shaped prophase nuclei, radiate from ill-definedspindle poles surrounded by plastids at metaphase and anaphaseand form a dense phragmoplast array during telophase. Pre-prophasebands are absent and it may be that the nuclear equator determinesthe plane of division in gemmiferous filaments. Chloroplastdivision, associated with extremely transient plastid-dividingrings, takes place during interphase. Lateral branches of thegrowing filaments arise from subapical cells by reiterationof the first division mechanism. Immediately following the proliferative divisions, which takeplace in cells measuring only 5-6 µm in diameter, oilbodies suddenly appear as flat pleomorphic cisternae associatedwith endoplasmic reticulum and occasional microtubules. Theircontents are electron-transparent apart from scattered osmiophilicdroplets. Throughout their ontogeny the oil bodies are closelyassociated with cytoplasmic lipid bodies but there is no evidenceof fusion. The nascent oil bodies swell rapidly to their finaldiameter, become ovoid to spherical in outline and are eventuallysuspended by fine cytoplasmic bridges within the vacuoles. Thelatter rapidly increase in size together with an expansion ofthe cells themselves until these reach their final diameterand length. The final event in gemma maturation is an endogenousdivision with the formation of a new internal wall along a phragmosome.Separation of the bicellular gemmae proceeds basipetally andinvolves the appearance of an electron-transparent line alongthe middle lamella in the cross walls, which often develop convexthickenings, and severing of the plasmodesmata. After theirliberation shallow scars are visible on the leaf surface underthe SEM. Gemma maturation sees a marked increase in the electron-opacityof the walls and dense staining of these together with Golgivesicles with the periodic acid/thiocarbohydrazide/silver proteinatetest for non-cellulosic carbohydrates. This change in wall chemistryand ultrastructure may be related to the fact that the maturinggemmae become extremely water repellant and are probably dispersedeither on the surface of water films or in the air.Copyright1995, 1999 Academic Press Bryophyta, gemmae, liverwort, microtubules, morphogenesis, oil bodies, polar growth, vegetative reproduction     

Ultrastructural Changes in Leaves of Cichorium during Somatic Embryogenesis

A detailed electron microscopy study of early cellular eventsduring somatic embryogenesis in leaves of Cichorium is described.Leaves on in-vitro grown plantlets were sectioned and put at35°C, in darkness, in an agitated liquid induction medium.No sign of embryogenic predetermination, such as thick cellwall, dense cytoplasm and enlarged nucleus, could be seen inany cell before treatment. Perivascular cells were the firstto react. Addition of glycerol (330 mM) allowed the arrest ofembryogenic cells at an activated stage. The main events werea thickening of the wall, with extracellular secretion and anaccumulation of Ca²⁺ in the vacuole, demonstrated by an antimonateprocedure. After 5 d, leaves were transferred to glycerol-freemedium where multicellular proembryos could be observed. Theyshowed reduced vacuoles, cortical microtubules, numerous multivesicularbodies and lipid globules. The embryoid cells were lined alongthe mesophyll lacunae by an extracellular secretion with a tubularstructure; histochemical tests proved its complex lipo-glyco-proteicnature.Copyright 1993, 1999 Academic Press Cichorium, extracellular tubular protein, somatic embryogenesis, vacuolar calcium     

天麻种子／原球茎和营养繁殖茎被菌根真菌定殖后的细胞分化

天麻（Gastrodia elata Bl.)种子可与紫萁小菇（Mycena osmundicola Lange),兰小菇（M.orchidicola Fan et Guo)等一类小菇属真菌共生萌发形成原球茎。侵入种皮的菌丝集结在柄状细胞外周的胚柄残迹中,首先侵入胚的柄状细胞,然后自柄状细胞侵入其他原胚细胞。原胚细胞发生功能分化,形成菌丝结细胞和消化细胞,初被菌丝定殖的原胚细胞具有消化菌丝的功能,随后,部分原胚细胞逐渐被菌丝充满,充育成菌丝结细胞。菌丝由菌丝结细胞进一步侵入消化细胞后最终被消化。由原球茎分化形成的营养繁殖茎（以下简称营繁茎）进一步被蜜环菌（Armilariella mellea(Vahl.Fr.)Karst.)定植,蜜环菌与紫箕小菇的菌丝同时存在于营繁茎中,但两菌相遇时都停止蔓延,互不交错侵染。     

Cytodifferentiation of the Seeds (Protocorms) and Vegetative Propagation Corms Colonized by Mycorrhizal Fungi

The stipecell, subepidermal parenchyma cells and inner cortical parenchyma cells were differentiated from Gastrodia elata Bl. seed and protocorm cells when they were colonized by the fungal hyphae of Mycena osmundicola Lange and M. orchidicola Fan et Guo. The hyphae aggregated in the suspensor remnant surrounding stipecell, primarily penetrated the stipecell, and then colonized the embryo of seed. Stipecell is the unique invading site of the hyphae. Subepidermal parenchyma cells containing pelotons of hyphae is also a kind of passage cells of hyphae, but, when primarily colonized by hyphae, they can degenerate a little of hyphae. The hyphae colonizing inner cortical parenchyma cells were totally degenerated, and the function of inner ocrtical parenchyma cells is digestive. The vegetative propagation corms, which differentiated from protocorms, were recolonized by Armilariella mellea (Vahl:Fr.) Karst., and the hyphae of A. mellea and M. osmundicola were found in the same cell, but there is a layer of cells uncolonized by mycorrhizal fungal hyphae. This means the two fungal species can not crisscross colonize the cell of G. elata.     

Relation of beet yellows virus to the phloem and to movement in the sieve tube

In minor veins of leaves of Beta vulgaris L. (sugar beet) yellows virus particles were found both in parenchyma cells and in mature sieve elements. In parenchyma cells the particles were usually confined to the cytoplasm, that is, they were absent from the vacuoles. In the sieve elements, which at maturity have no vacuoles, the particles were scattered throughout the cell. In dense aggregations the particles tended to assume an orderly arrangement in both parenchyma cells and sieve elements. Most of the sieve elements containing virus particles had mitochondria, plastids, endoplasmic reticulum, and plasma membrane normal for mature sieve elements. Some sieve elements, however, showed evidence of degeneration. Virus particles were present also in the pores of the sieve plates, the plasmodesmata connecting the sieve elements with parenchyma cells, and the plasmodesmata between parenchyma cells. The distribution of the virus particles in the phloem of Beta is compatible with the concept that plant viruses move through the phloem in the sieve tubes and that this movement is a passive transport by mass flow. The observations also indicate that the beet yellows virus moves from cell to cell and in the sieve tube in the form of complete particles, and that this movement may occur through sieve-plate pores in the sieve tube and through plasmodesmata elsewhere.     

Polar Regeneration in Excised Roots of Taraxacum officinale Weber: A Light and Electron Microscopic Study

The day 0 secondary phloem of Taraxacum officinale root segmentscontains wide bands of parenchyma alternating with thin cylindersof conducting tissue composed of discreet conducting strands.At day 1 in the inner distal phloem (and by day 2 proximally)the initially-flattened nuclei of some parenchyma cells becomerounded, more densely stainable and a few have migrated fromthe peripheral cytoplasm to a suspended position in the vacuole.Cell division occurs asynchronously and gradually extends tothe midphloem. By day 4 nodules of primary meristematic cellsoccur proximally and numerous young leaves are visible externallyat days 5–6. Distally, callusing of the phloem is moreextensive and links with that developing from the secondaryxylem. Proximally adventitious buds form and these are bothmore abundant and quicker growing than the distally locatedadventitious roots. Although proliferation is initially mainly confined to the companioncells it increasingly involves activation of the parenchymatissue. These cells undergo a cytological de-differentiationwith the daughter cells showing a progressive decrease in cellsize and vacuome (with cytoplasmic strands, perhaps indicativeof lysosomal activity, often visible in the vacuoles), accompaniedby an increase in nucleolar size and cytoplasmic density.     

Localization of Phloem Oxidases

Among oxidases, cytochrome oxidase has been localized in mitochondria of all phloem cells, catalase has been visualized in parenchyma peroxisomes and peroxidase has been localized in cell walls and in several cell organelles. In angiosperms, peroxidase is present in all phloem cell walls; it is sensitive to cyanide inhibition excepted in sieve areas and around plasmodesmata between sieve tubes and companion cells. In some species, this cyanide resistant oxidasic activity can be localized without exogenous H₂O₂. Peroxidase is localized on ribosomes, inside vacuoles, on the tonoplast and often on the plasmalemma in companion cells and differentiating sieve elements. In young sieve cells some dictyosomes can exhibit a strong peroxidasic activity. In mature parenchyma cells peroxidase can be associated with ER cisternae but not with vacuoles.     

Comparative Structure of Companion Cells and Phloem Parenchyma Cells in Mimosa pudica L.

The phloem of Mimosa pudica L. furnishes an example of definablediversification of the parenchymatic members of the tissue intocompanion cells and parenchyma cells. The companion cells havedense protoplasts which contain the typical organelles of plantcells, including chloroplasts and many ribosomes. The sieveelements and companion cells are interconnected by numerousbranched plasmodesmata. The companion cells degenerate whenthe associated sieve elements cease to function. The parenchymacells have less dense protoplasts than the companion cells.In many parenchyma cells the rough endoplasmic reticulum assumesa tubular form, and bundles of microfilaments are present. Thecytoplasmic ribosomes occur in groups apparently held togetherby fibrils. Chloroplasts, mitochondria (some are exceptionallylong), dictyosomes, microbodies, and microtubules are the othercell components. Whether the parenchyma cells are ontogeneticallyrelated to the sieve elements or not, they do not degeneratewhen the sieve element ceases to function.     

Root Cap Structure in Isoetes macrospora Dur

Root meristem cells of Isoetes macrosporausually have one plastidwhich is associated with the prominent nucleus, numerous ribosomesand mitochondria, and small vacuoles. During mitosis each plastidappears to replicate so that each daughter cell contains oneplastid. The cell walls of the meristem cells are traversedby numerous plasmodesmata. Central cells of the root cap lackdistally displaced plastids but have one or more amyloplastsassociated with the nucleus. These cells also contain largeprotein deposits. Peripheral root cap cells are characterizedby being vacuolated, and by possessing a few dictyosomes andprotein deposits. They appear to be sloughed infrequently. Isoetes macrospora Dur, root cap, protein bodies, ultrastructure     

STRUCTURE OF THE VASCULAR PARENCHYMA IN THE STEM OF LYCOPODIUM LUCIDULUM

At maturity the vascular cylinder of the stem of Lycopodium lucidulum contains two distinct types of parenchyma cells, one which is always associated with sieve cells, the other with tracheids. The remaining parenchyma cells have characteristics intermediate between the two extremes. The most conspicuous feature of the sieve cell-associated parenchyma cell is the very dense appearance of its protoplast, due to a high ribosome population and absence of large vacuoles. The large, ramifying nuclei of these cells have numerous connections with the endoplasmic reticulum (ER). The tracheid-associated parenchyma cells, which are light in appearance, contain many small vacuoles and a relatively small ribosome population. These cells also contain relatively small nuclei and considerable ER cisternae. The parenchymatous elements which have characteristics intermediate between sieve cell- and tracheid-associated parenchyma may or may not be contiguous to the sieve cells or tracheids. An intergradation in wall thickness occurs among parenchyma cells of the vascular cylinder, the thicker-walled cells being adjacent to the sieve cells, the thinner-walled ones next to the tracheids. An intergradation also occurs in the frequency of plasmodesmata between the various parenchyma cells. The closer parenchyma cells are to the sieve cells the greater the number of connections between them. No plasmodesmata were found between the tracheid-associated parenchyma cells.     

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     

The Digestive Glands of Pinguicula: Structure and Cytochemistry

The digestive glands of the carnivorous genus Pinguicula havethree functional compartments, (a) a basal reservoir cell, (b)an intervening cell of endodermal character and (c) a groupof secretory head cells. The gland complex is derived from asingle epidermal initial. The reservoir cell, which is richin Cl^– ions, is highly turgid before discharge; it islinked by plasmodesmata to the surrounding epidermal cells,and is ensheathed by a pectin-rich inner wall layer. The endodermalcell is bounded by a Casparian strip to which the plasmalemmais tightly attached; it contains abundant storage lipid andnumerous mitochondria. The head cells of the developing glandhave labyrinthine radial walls of the transfer-cell type, theingrowths being composed of pectic polysaccharides. The boundingcuticle is discontinuous, although lacking well-formed pores.Mitochondria are numerous, with well-developed cristae; theplastids are large and ramifying, and invested by ribosomalendoplasmic reticulum. Dictyosomes are sparse, and where theyoccur, are associated with coated vesicles. Ribosomal endoplasmicreticulum is moderately abundant in the head cells, and so alsoare free ribosomes. Optical and electron microscopic localizationmethods indicate that the digestive enzymes are synthesizedin the head cells and transferred both into the vacuoles andinto the walls. There is no evidence of a granulocrine modeof secretion, and the transfer seems to be initially by directperfusion through the plasmalemma. During the final phase ofmaturation of the head cells they suffer a form of autolysis,vacuoles, cytoplasm and wall becoming confluent as all of themembranes of the cell undergo dissolution. The gland head isthus, in effect, simply a sac of enzymes at the time of theultimate discharge. Pinguicula, carnivorous plant, insectivorous plant, enzyme secretion, digestive gland     

Electron microscopy of vesicular-arbuscular mycorrhizae of yellow poplar. III Host-endophyte interactions during arbuscular development.

Scanning- and transmission-electron microscopy were used to examine developing and mature functional arbuscules in mycorrhizal roots of yellow poplar. Arbuscules developed from intracellular hyphae which branched repeatedly upon penetration into the host cells. Intermediate and late stages of developemnt were characterized by the production of numerous, short, bifurcate hyphae throughout the arbuscule. Mature arbuscules exhibited a coralloid morphology which resulted in a considerable increase in the surface area of the endophyte exposed within the host cells. Distinctive ultrastructural features of arbuscular hyphae included osmiophilic walls, nuclei, abundant cytoplasm, glycogen, and numerous small vacuoles. All arbuscular components were enclosed by host wall material and cytoplasm during development and at maturity. In infected cells, host nuclei were enlarged and the cytoplasm associated with the arbuscular branches typically contained abundant mitochondria, endoplasmic reticulum, and proplastids. Ultrastructural observations suggested that nutrient transfer may be predominantly directed toward the fungal endophyte during arbuscular development and while mature arbuscules remain functional.