Multizooidal Budding in Parasmittina trispinosa (Johnston) (Bryozoa,Cheilostomata)

Two principally different wall types occur in the bryozoan colony: Exterior walls delimiting the super-individual, the colony, against its surroundings and interior walls dividing the body cavity of the colony thus defined into units which develop into sub-individuals, the zooids. In the gymnolaemate bryozoans generally, whether uniserial or multiserial, the longitudinal zooid walls are exterior, the transverse (proximal and distal) zooid walls interior ones. The radiating zooid rows grow apically to form “tubes” each surrounded by exterior walls but subdivided by interior (transverse) walls. The stenolaemate bryozoans show a contrasting mode of growth in which the colony swells in the distal direction to form one confluent cavity surrounded by an exterior wall but internally subdivided into zooids by interior walls. In the otherwise typical gymnolaemate Parasmittina trispinosa the growing edge is composed of a series of “giant buds” each surrounded by exterior walls on its lateral, frontal, basal and distal sides and forming an undifferentiated chamber usually 2–3 times as broad and 3 or more times as long as the final zooid. Its lumen is subdivided by interior walls into zooids 2–3, occasionally 4, in breadth. This type of zooid formation is therefore similar to the “common bud” or, better-named, “multizooidal budding” characteristic of the stenoleamates but has certainly evolved independently as a special modification of the usual gymnolaemate budding.     

Body wall morphology of Pentapora foliacea (Ellis and Solander) (Bryozoa,Cheilostomata)

The ascophoran Pentapora foliacea was studied from epoxy sections of skeletal and soft (hard-soft) tissues. The basal wall is double, indicating the colony grew as two independent layers, back to back. The structure of the vertical walls and interzooidal communication organs indicates that zooids were budded in the usual way as in most encrusting cheilostomes. Secondary layers of the frontal wall are of acicular aragonite. The ovicell develops as a flattened cuticular bladder in early ontogeny; the aragonitic layer of the frontal wall later engulfs it. A median vesicle, an evagination of the vestibular wall, is present but the eggs may be supplied with sufficient yolk to nurture the embryo. The overall ovicell structure is similar to that of hyperstomial ovicells in other cheilostomes.     

Regressive astogenese beiNudonychocella n.g.n.sp. und anderen bryozoen aus der tuffkreide von maastricht

Nudonychocella nuda n.g.n.sp. from the Maastrichtian chalk tuff is a cheilostome bryozoan species which shows totally opened “membranimorph” zooecia and avicularia. Its ancestrula, however, displays — likeOnychocella Jullien — semicircular opesiae and normally formed calcified frontal walls (cryptocysts), revealing its lineage from this genus which belongs to the Coilostega. This is suggested to be a matter of regressive astogeny, unknown until now in bryozoology, during which the calcareous cryptocyst, being characteristic for that genus, was totally reduced.Onychocella cyclostoma (Goldf.) andO. koninckiana (v. Hagenow) also show the tendency to reduce their calcareous frontal walls which are still widely normal during the neonastic stage of the colony. InCastanopora bipunctata (GOLDF.), however, the cribrimorph frontal shield may disappear totally. A satisfactory explanation of this phenomenon may be that the neonastic stage characterising the peripheral growing tip of the colony with its incompletely calcified frontal walls of the zooids dominated the whole colony, induced by a stop of the calcification of the cryptocyst, with the exception of the ancestrula, the result of which being a secondary membranimorph stage, characteristic for the Malacostega. The reduction of the calcification does not contradict to Dollo’s rule concerning the irreversibility of the evolution. Consequences arising for taxonomic questions and phylogenetic research are discussed.     

Calcite and aragonite distributions in the skeletons of bimineralic bryozoans as revealed by Raman spectroscopy

Abstract. Bryozoans are among a diverse range of invertebrates capable of secreting calcium carbonate skeletons. Relatively little is known about biomineralization in bryozoans, despite the importance of understanding biomineralization processes for nanotechnology and the threats imposed by ocean acidification on organisms having calcareous skeletons. Ten species of cheilostome bryozoans that are reported to have bimineralic skeletons of calcite and aragonite are studied here using Raman spectroscopy. This technique allowed identification of the two mineral phases at submicron spatial resolution, allowing the distributions of calcite and aragonite within bryozoan skeletons to be determined with unprecedented precision. Confirming previous findings based on the use of chemical stains, most of the bimineralic species analyzed exhibited a calcitic skeletal framework, composed of basal, vertical, and inner frontal walls, having aragonite deposited subsequently onto the outer surfaces of the frontal walls. In one species ( Odontionella cyclops ), aragonite formed the superstructure above the autozooids, and in two others, traces of aragonite were detected on the undersides of the frontal shields. Using Raman spectroscopy, it was possible for the first time to determine the mineralogy of small-scale structures, including orificial rims, condyles and hinge teeth, avicularian pivotal bars and rostra, and ascopore rims and sieve plates. Even when surrounded by aragonitic frontal shields, these structures were found typically to be calcitic, the two exceptions being the aragonitic avicularia of Stylopoma inchoans and O. cyclops . Unexpectedly, the first-formed part of the basal wall at the distalmost growing edge of Pentapora foliacea was found to consist mainly of aragonite. This may point to a precursory phase of biomineralization comparable with the unusual mineralogies identified previously in the earliest-formed skeletons of members of some other invertebrate phyla.     

Comparative Studies of Ovicell Anatomy and Reproductive Patterns in Cribrilina annulata and Celleporella hyalina (Bryozoa: Cheilostomatida)

Investigations of the common boreal-arctic cheilostomate bryozoans Cribrilina annulata and Celleporella hyalina have shown that the two species possess similar ovicell structures and reproductive patterns. Both species are characterized by frontal dwarf ovicellate zooids, that are female autozooidal polymorphs in C. hyalina and simultaneous hermaphroditic autozooids in C. annulata. The latter species in addition has ovicellate autozooids of the usual type. Each ovicell is formed from a maternal zooid only, and its cavity is lined by the outer hemispherical fold (ooecium) and the distal zooidal wall. The coelomic cavity of the ooecium is separated from the body cavity of the maternal zooid by a transverse wall with simple pores. Each pore is closed by a cell plug, and the ooecia may be considered as kenozooids. Each oocyte is accompanied by a single nurse cell that degenerates after ovulation. The eggs are macrolecithal in C. annulata and microlecithal in C. hyalina, and the former species is a non-placental brooder whereas the latter forms a placenta. Fertilization is precocious. Possible mechanisms of sperm entry as well as oviposition are discussed. The literature concerning ovicell structure and development in cheilostomates is analysed. It is proposed that the brood chamber of cribrimorphs evolved by a fusion of costae and a reduction of the daughter zooid in ancestral forms. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

The body wall of cheilostome Bryozoa. II. Interzoidal communication organs

Communication organs (septulae) of cheilostome Bryozoa are more complex than perviously believed. Annuli, present only in lateral septulae, are thickenings of the intercalary cuticle. Each communication pore is filled with a ring-like “pore cincture,” through which project a pair of “special cells.” Septulae of all species examined (10 species from 6 families) can be considered modifications of the same structure, varying only in degree of calcification and number of communication pores. External walls, including basal and lateral walls, are best defined as reinforcements of the ectocyst, which is derived by intussusception from the primary cuticle of the ancestrula. The lateral ectocyst must be considered a double layer formed by invagination of the distal ectocyst. Internal walls are developed by apposition from inner parts of the ectocyst; they include pore plates and transverse walls. External walls are laid down first. Lenticular masses develop unilaterally on the uncalcified lateral ectocyst; the pore plate develops by apposition from the interior part of the ectocyst. Depending on the species, the pore plate may or may not be calcified at the time of its formation. Communication pores are formed when the developing pore plate abuts against embryonic special cells. The septular ectocyst never calcifies; it breaks down when the pore plate is complete. Some ascophorans undergo “reparative budding,” in which new zoids are formed within dead zoecia. Hollow, ectocyst-covered buds lined with blastemic epithelia are produced from septulae of live zoids; adjacent buds may fuse. These findings are consistent with the view that lateral septulae are aborted zoids and that pore plates represent transverse walls.     

Microarchitecture of Body Wall of Extant Cyclostome Ectoprocts

Scanning electron microscope and light microscope studies ofthe body wall in cyclostome stenolaemate ectoprocts show thatthis wall has a distinctive microarchitecture. The microstructuresconsist of a linear series of longitudinal canals in the centralregion. Adjoining regions on either side have laminae of overlappingcalcareous tablets. In some cyclostomes at certain stages ofgrowth, laminae may show a less orderly meshwork pattern nearthe canals. The laminate layers of calcareous tablets are enclosedin an organic matrix and are penetrated by tubuli which passfrom the inner part of the body wall to the canals and alsolink the canals. The canals with organic and mineralized matricesopen into pores in the body wall. This elaborate meshwork andcanal and pore system provide the framework for growth and resorptionof the body wall and facilitate transport of cellular materialsfrom one zooid to another in a colony. Calcification at thedistal part of a colony seems to proceed in a series of stagesaround sites of calcification. The pattern of the body wallof the cyclostome ectoproct does not parallel a molluscan patternof skeletal growth as previously proposed. The hypothesis ofa hypostegal coelom in some cyclostome ectoprocts is rejected.     

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     

Skeletal alterations and polymorphism in a Mediterranean bryozoan at natural CO<Subscript>2</Subscript> vents

Colonies of the cheilostome bryozoan Schizoporella errata were grown at a site near Ischia Island (Tyrrhenian Sea, Italy) where volcanogenic CO₂ emissions lower seawater pH to 7.76, simulating levels of ocean acidification predicted for the end of the present century. Compared with colonies from a control site (mean pH = 8.09), putative defensive polymorphs (avicularia) were significantly fewer, and retarded growth of zooidal basal and lateral walls was evident at the low pH site. The lower proportion of avicularia suggests a switch in resource allocation away from defence to favouring rapid growth. In addition, corrosion of the skeleton was observed in both new and old zooids at the low pH site, and feeding zooids were slightly smaller but had larger orifices for the protrusion of feeding lophophores. These findings corroborate previous studies demonstrating potential dissolution of carbonate skeletons in low pH seawater, while providing new insight into the possible ability of colonial species to respond to ocean acidification by adjusting resource allocation between zooids of different types.     

Ultrastructure and Development of Oil Idioblasts in Annona muricata L.

The ultrastructure and development of oil idioblasts in theshoot apex and leaves in Annona muricata L. are described, andthree arbitrary developmental stages are distinguished: cellsin which no additional cell wall layers have been depositedagainst the initial primary cell wall, possessing an electron-translucentcytoplasm and distinct plastids which lack thylakoids (stage1); cells in which a suberized layer has been deposited againstthe primary wall (stage 2, the cytoplasm resembles that of thepreceding stage), and cells in which an additional inner walllayer has been deposited against the suberized layer, whichincreases in thickness with development (stage 3). In this stagean oil cavity is formed, surrounded by the plasmalemma, andattached to a bell-like protrusion of the inner wall layer,the cupule. A complex membranous structure occurs next to thecupule. Smooth tubular endoplasmic reticulum (ER), appearingas linearly arranged tubules, and groups of crystalline bodieswith an almost hexagonal outline are present. The final stagewas further subdivided into three subgroups (a, b, c) basedon the extent of the oil cavity, its contents, and the compositionof the cytoplasm, and increasing thickness of the inner walllayer. The oil is probably synthesized in the plastids, releasedinto the cytoplasm, and then passed through the plasmalemmasurrounding the oil cavity. Oil idioblasts, Annona muricata L., suberized layer, inner wall layer, oil cavity, cupule, smooth tubular ER, crystalline bodies     

Silicon Distribution and Anatomy of the Grass Rhizome, with Special Reference to Miscanthus sacchariflorus (Maxim.) Hackel.

Rhizome anatomy is described for Miscanthus sacchariflorus (Maxim.)Hackel. Solid silica deposits, detected as elemental siliconby electron-probe microanalysis and energy-dispersive X-rayanalysis, are confined to cell walls of three concentric zonesconsisting of the uniseriate epidermis, and parenchyma layersaround the cortical air lacunae, and the central cavity, respectively.Si is localized in outer tangential walls of the epidermis,while occurring in all walls of nucleated, parenchyma cellsforming the two internal zones. In comparison, the root exhibitsonly one Si zone. Rhizome Si distribution more closely resemblesthat for Phragmites australis, than for related members of theAndropogoneae. P. australis similarly exhibits aerenchyma anda central cavity. Thus, internal anatomy may strongly influencesilicon distribution. A comparison of taxa of four tribes indicatesthat epidermal wall deposition is common, followed by specificinternal localization in up to three zones of perivascular tissues. Silicon accumulation occurs early in the epidermis of the youngapex of M. sacchariflorus, decreasing sharply across an internodetransection. In comparison, the oldest, basal internodes exhibitvery high Si X-ray counts in each of the three zones, the highestoccurring in the most internal zone around the central cavity.Early Si mobilization in the rhizome apex may resist shearingand abrasion during horizontal growth extension, while depositsbordering aerenchyma of older internodes may resist compression. Miscanthus sacchariflorus (Maxim.) Hackel, plume grass, rhizome, silicification, anatomy, aerenchyma     

The Hydraulic Pathways in Nicotiana glauca (Grah.) and Tradescantia virginiana (L.) Leaves, and Water Potentials in Leaf Epidermes

The hydraulic conductances of leaves of a species which exhibitsstomatal responses to humidity (Nicotiana glauca) are significantlylower than the conductances in a species which does not exhibitsuch responses (Tradescantia virginiana). This difference couldat least partly account for their difference in stomatal responseto humidity. In both species, the hydraulic conductance betweenthe leaf bulk and its epidermis is much lower than the conductancein any other part of the pathway. The apparently conflictingresults, reported in recent literature, on the hydraulic conductancesand water pathways in leaves are reinterpreted, and shown tobe due to misinterpretation of results. The recently publishedcriticisms of a technique used to measure hydraulic conductivityare commented on and refuted. An examination of the factors that influence the water potentialat the sites of evaporation from the inner walls of the epidermisnear stomatal pores showed that the water potential at thesesites is lower than the bulk epidermal water potential. Thewater potential at these sites changes in a complex way as stomatalaperture changes. As it is reduced the ratio of: ‘waterpotential at sites of evaporation on the inner walls of theepidermis near stomatal pores/bulk leaf water potential‘increases. The positive feedback effect of this phenomenon,which tends to keep stomatal water potential constant as thestomata close and therefore enhances closure, and two other‘passive’ positive feedback effects on the waterpotential at sites of evaporation near stomata that have beenreported in the literature are briefly discussed. Nicotiana glauca (Grah.), Tradescantia virginiana (L.), sub-stomatal cavities, peristomatal evaporation, stomata, humidity response, leaf hydraulic conductance, water potential     

The Floral Nectaries of Hibiscus rosa-sinensis: 1. Development of the Secretory Hairs

Floral nectaries of Hibiscus rosa-sinensis occur on the lowerinner side of the fused sepals and each one consists of numerous(50000–55000) secretory hairs, occupying a cylinder-likezone completely lining the inner side of the sepals. Each hairoriginates from a single protodermal mother cell and, at maturity,it is built up of a basal cell, a stalk, 35–40 intermediatecells and a tip secretory cell. Development of protodermal cellsinto secretory hairs is asynchronous, the first cells to initiatedevelopment being those situated in the lowermost part of thecylindrical zone, and development progressing upwards. Volume increase of protodermal mother cells initiating developmentis accompanied by cell polarization manifested by organelledisplacement towards the apical region. Secretory hairs areformed through a sequence of transverse and, later on, anticlinaldivisions. Divisions of apical cells are preceded by well definedpre-prophase microtubule bands, which foreshadow the plane ofthe forthcoming division and predict with accuracy the sitesof parental walls where the new cell plate fuses at cytokinesis. Stalks consist of either one or two cells. Two-celled stalksoccur in 40 per cent of secretory hairs and derive from a transversedivision of one stalk cell; the wall formed is always depositedparallel to the proximal and distal walls, but never to thelateral ones. The significance of this mode of division is discussedin relation to the fact that lateral walls are entirely impregnatedwith a cutin-like material that blocks apoplastic movement ofsolutes. Hibiscus rosa-sinensis, nectaries, development, preprophase microtuble bands, stalk cells     

Unchanged Molecular-Weight Distribution of Xyloglucans in Outer Tissue Cell Walls along Intact Growing Hypocotyls of Squash (Cucurbita maxima Duch.) Seedlings

The changes in the mechanical properties and compositions ofcell walls in outer and inner tissues were investigated alongthe hypocotyls of squash (Cucurbita maxima Duch.) seedlings.The endogenous growth capacity decreased and the minimum stress-relaxationtime (T_O) of cell walls in outer tissues increased from theapical to the basal region of hypocotyls. A high correlationwas observed between values of To in outer tissues and endogenousgrowth (r=–0.99). The values of T_O in inner tissues didnot change from the apical to the basal region of hypocotyls. In outer tissues, the levels of neutral sugars in pectin decreasedconsiderably from the apical to the basal region of hypocotyls.However, relative amounts of hemicellulose showed little differencealong the hypocotyls. Levels and molecular weights of hemicellulosicxyloglucans in outer tissues were about 2-3 times greater thanthose in inner tissues. The amount of xyloglucans in outer tissuesincreased in the middle region of hypocotyls, and xyloglucansin upper and basal regions had similar molecular weights. Bycontrast, in inner tissues, amounts of cell-wall material decreasedtoward the basal region. Amounts and molecular weights of hemicellulosicxyloglucans also decreased along the hypocotyls. These results clearly show that cell-wall metabolism duringaging of intact growing stem tissues differs markedly betweenouter and inner tissues, and the absence of a simple relationship between the molecular weights of xyloglucans and the mechanicalproperties of the cell walls in outer tissues indicates thatthe changes in the mechanical properties of the cell walls inintact growing tissues cannot be explained only by the molecularweights of xyloglucans. Thus, the regulation of the mechanicalproperties of cell walls in intact growing stems may be somewhatdifferent from that in auxin-treated stem sections, in whichauxin promotes the depolymerization of xyloglucan molecules. (Received November 28, 1991; Accepted November 16, 1992)     

Sieve Elements of Minor Veins in the Leaves of Beta vulgaris L.

End walls of sieve elements of minor veins of the leaves ofBeta vulgaris L. do not contain the multi-perforate sieve plateswhich typically occur on the end walls of sieve-tube membersof major veins. Instead, both end and side walls of the sieveelements of minor veins contain scattered pores which may occursingly or in small numbers. These pores are similar to thosewhich are grouped in sieve plates of major veins in size, possessionof callose and plugs of filaments. In addition to these pores,there are tubular connections 0.1 µ in diameter throughcharacteristically thickened parts of the cell wall betweensieve cells and companion cells. Sieve elements of minor veinsdiffer from those of major veins in structure as well as infunction.     

Electron-probe Microanalysis Studies of Silicon in the Elongating Basal Internodes of Avena sativa (L.), Hordeum sativum (Jess.) and Triticum aestivum (L.)

Silicon deposits in the elongating basal internodes of almostmature, field grown specimens of Avena sativa, Hordeum sativumand Triticum aestivum were investigated using electron-probemicroanalysis. In A. sativa and H. sativum silicon was foundto be confined to the cells in the endodermal layer, being presentwithin the inner tangential and radial walls, and occasionallyin the outer tangential wall. In T. aestivum some silicon wasalso located in walls of cells surrounding the vascular bundles. The anatomy of the internodal tissues is discussed for the threespecies from light micrographs. The endodermal layer is discontinuousin A. sativa and H. sativum, in the former species it partlyextends around individual vascular bundles. In T. aestivum itforms a complete cylinder around the stelar region and alsoshows considerably more thickening of the inner tangential wallthan in the other species. The results are discussed in relation to the anatomy of theinternodal tissues and the possible function of silicon in theendodermis. Avena sativa L., Hordeum sativum Jess, Triticum aestivum L., oat, barley, wheat, silicon deposition, electron-probe microanalysis     

Morphology and ultrastructure of the sensory spine,a presumed mechanoreceptor of Sphaeroma hookeri (Crustacea,Isopoda), and remarks on similar spines in other peracarids

The isopod Sphaeroma hookeri and many other isopods and peracarids have a sensory spine with laterally inserting sensory hair, positioned in the apical region of the propodal palm of pereopod 1. This spine is innervated by five to eight sensory cells (each giving rise to one cilium) the dendrites of which can be divided into an inner and outer dendritic segment. The cilia are surrounded by an extracellular, electron-dense dendritic sheath. Thirteen enveloping cells are present. The outer dendritic segment (structure beyond the basal bodies) contains two receptor lymph cavities; the inner one lying within the dendritic sheath is homologous with the inner receptor lymph cavity of insects. Scolopales, or tubular bodies, are lacking; their function is probably accomplished by the dendritic sheath. Apically the sensory hair does not have a pore, and the spine is heavily sclerotized. The inner dendritic segment begins with a basal body from which rootlets of different length and thickness extend into the dendrite. In the latter is an accumulation of vesicles. The dendrites keep close contact with other dendrites and the enveloping cells by desmosomal membrane structures. The possible importance of the sensory spine for phylogenetic studies is discussed.     

How much can Conus swallow? observations on molluscivorous species

Feeding of three species of molluscivorous Conus, C. textile,C. bandanus and C. omaria, was studied in aquaria. Conus spp.are able to kill and remove from the shell prey larger thanthemselves. Also, Conus swallowed prey with weight up to halfthat of the predator. Estimates suggest that molluscivorousspecies of Conus are probably able to swallow prey with a shellvolume reaching 85% of that of the predator, depending on theshape of the prey's body. It is confirmed that the thinningof the inner shell walls in Conus is connected with the abilityto swallow voluminous prey. Digestion of prey occurs in boththe oesophagus and stomach. (Received 9 August 2006; accepted 4 January 2007)     

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     

A Study of Stelar Ultrastructure in the Heterosporous Water Fern Marsilea quadrifolia L

Sieve tube elements occur in the rhizomes and petioles of Marsileaquadrifolia. These are either thick walled with compound sieveplates in oblique end walls or thin walled with simple sieveplates in transverse end walls. Vessels are restricted to themetaxylem in the roots where the phloem contains sieve cellsonly. The sieve pores are invariably callose lined and as inother pteridophytes, excepting the Lycopsida, refractive spherulesare ubiquitous in the sieve elements of Marsilea. The luminaof the protoxylem tracheary elements in the rhizomes and petiolesare occluded by tyloses but probably remain functional in theroots. Pericycle cells backing on to the root protoxylem armspossess wall ingrowths. Transfer cells are however absent fromthe vascular tissue of the rhizomes and leaves. It is suggestedthat their presence in the root pericycle is related to theretrieval of ions from the xylem sap which may be particularlycritical in water plants. The incidence of transfer cells incryptogams appears to be far more sporadic than in angiosperms.The root endodermis of Marsilea possesses a casparian stripand abundant vacuolar tannin deposits. Plasmalemmasomes arenumerous adjacent to the pericycle transfer cells. vascular ultrastructure, Marsilea quadrifolia L, transfer cells, sieve tube elements, tyloses