Skeletal Ultrastructure in Some Articulate Cyclostome Bryozoans

The ultrastructure of the calcareous skeleton is described in 11 species of articulate cyclostome bryozoans with elastic joints. Ten species have interior walls comprising semi-nacreous and pseudofoliated fabrics without a precursory granular layer. Exterior walls consist of outer, finely granular and planar spherulitic layers, succeeded by semi-nacreous and pseudofoliated fabrics like those of interior walls. Outer fabrics are calcified as longitudinal strips, each corresponding to a planar sphcrulitic unit. Articulation surfaces comprise ring diaphragms of very fine granular fabric with concentric laminations. The semi-nacre of walls adjacent to ring diaphragms contains minute holes. Crisulipora occidentalis is unique in having interior walls of transverse fibres succeeded by pseudofoliated fabric, articulation surfaces festooned with deep pits but lacking well-differentiated ring diaphragms, and pseudopores containing sieve-like closure plates. The ultrastructure of most articulates resembles tubuliporine cyclostomes with dominantly semi-nacreous walls, although the lack of precursory granular fabric in the interior walls and the presence of subcircular tablets of semi-nacre (without six-fold sectoring) may be peculiar to articulates. In contrast, Crisulipora is more similar to other tubuliporines with transverse fibres. evidence which, together with other skeletal characters, suggests that Crisulipora evolved jointing independently of the rest of the articulate cyclostomes.     

Skeletal Ultrastructure in some Tubuliporine Cyclostome Bryozoans

The ultrastructure of the calcareous skeleton is described in twenty–one species of recent tubuliporine cyclostome bryozoans, using field emission SEM. The succession of skeletal fabrics in interior walls may be classified into four different fabric suites. The first–formed part of the calcitic skeleton in all species for which it has been observed is a precursory fabric of tiny, wedge–shaped crystallites. This is succeeded in about half of the species studied by a fabric of transverse fibres, followed by foliated fabric and often semi–nacre (fabric suite 1). Most of the remaining species lack transverse fibres and have interior walls largely comprising semi–nacre (fabric suite 2). A few species have skeletons consisting of predominantly distally–oriented, irregularly or regularly foliated fabric (fabric suite 3). A single species has a skeleton of proximally–oriented foliated fabric (fabric suite 4). Basal exterior walls in all species have a precursory fabric of tiny wedge–shaped crystallites without a strong preferred orientation, deposited directly upon the organic cuticle, followed by a layer of planar spherulitic structure, which in turn is succeeded by a similar fabric to that developed in the interior wall of the species concerned. Outermost layers of frontal exterior walls exhibit one of the following combinations of three fabrics: an outer layer of (1) finely granular or wedge–shaped crystallites; a thin dense granular layer followed by (2) distally accreting planar spherulitic fabric., or (3) obliquely accreting planar spherulitic fabric growing partly towards the midline of the frontal wall. Terminal diaphragms usually have outer layers dominated by planar spherulitic ultrastructure with centripetal growth directions. The fabric suites present in tubuliporines encompass most known fabrics found in the other cyclostome suborders and support the notion that this species–rich suborder occupies a central position in cyclostome evolution.     

Skeletal Ultrastructure of the Early Astogenetic Stages of Some Cyclostome Bryozoans

The calcific protoecia of cyclostome bryozoans have remarkably uniform skeletal ultrastructure in three suborders (Tubuliporina, Cancellata and Rectangulata). The basal wall or floor has a fine outer granular layer succeeded by planar spherulitic fabric, internally lined by irregular semi-nacre. The roof of the disc comprises an outer granular layer with an inner lining of semi-nacre continuous with that of the floor; planar spherulitic fabric is absent. Growth of the floor of the disc is initiated around the circular outer rim and continues centripetally to the centre; the inner lining has no prevailing growth direction. The gently domed roof also initiates around the outer rim, and grows in strips, which grow toward the centre, then distally toward the distal tube of the ancestrula which has a fully adult ultrastructure. Protoecia] ultrastructure is independent of adult ultrastructure. The uniformity of skeletal ultrastructure in cyclostomes corresponds with the close similarity of larvae and post-settlement metamorphosis in the order. The fabric suite of the protoecium resembles the skeletal ultrastructure of Palaeozoic stenolaemates. The primitive fabric condition is retained by some tubuliporines and cancellates. Complex multilayered fabric suites may have evolved in the Mesozoic by addition of new fabric types.     

Skeletal Ultrastructure in some Cyclostome Bryozoans of the Family Lichenoporidae

The ultrastructure of the skeleton is described in six species of lichenoporid cyclostome bryozoans using field emission SEM. Both interior walls (vertical, interzooidal walls, and brood chamber roofs and floors) and exterior walls (basal walls) are initially secreted as tiny wedge-shaped crystallites without a strong preferred orientation. These are seeded directly onto pre-existing crystallites in the case of interior walls, but onto the organic cuticle in exterior walls, the bases of the crystallites forming a tightly packed mosaic against the cuticle. With growth the wedges become longer, broader and relatively flatter, developing into platey crystallites. These crystallites grow predominantly distally (i.e. parallel to wall growth direction) and are closely imbricated in a foliated fabric. Local disruptions to this pattern occur, especially in association with crystallite division along “divergent zones”. The pattern also breaks down in old walls where crystallite growing edges become less evident and imbrication is poorly developed. Although conforming to this general model, some differences exist between species of lichenoporids, and in the patterns found in different parts of the skeleton (e.g. apertural spines). Lichenoporid ultrastructure differs from that of both cinctiporid and hornerid cyclostomes: notably, lichenoporids lack the layer of transverse fibres found in cinctiporids, and their predominant distal growth direction of crystallites contrasts with the proximal direction found in hornerids.     

Skeletal Ultrastructure in the Cyclostome Bryozoan Hornera

Abstract Scanning electron microscopy of calcified walls in two species of the cyclostome bryozoan Hornera has revealed previously undescribed details of skeletal morphology and growth. The calcitic interior walls of both H. robusta MacGillivray and H. squamosa Hutton have a laminated structure. Walls are extended at distal growing edges where the formation of new crystallites is concentrated and wall fabric is nacreous or semi-nacreous. New crystallites are seeded on the surface of existing crystallites as six-sided rhombs. At the centres of the rhombs in H. robusta there are often three ‘spikes' which point towards alternate sides of the rhomb. Screw dislocations resulting in spiral overgrowths are also common at these distal wall edges. Wall thickening occurs further proximally where walls develop a regularly foliated structure of imbricated laths growing towards the colony base. Although often thought to be ubiquitous in cyclostomes, the division of walls into three layers (an inner, primary layer flanked on both sides by secondary layers) is absent in Hornera. Wall ultrastructure contrasts strongly with the lamellar–fibrous–lamellar structure recently described from cinctiporid cyclostomes. The c-axes of the crystallites are orientated perpendicular to the wall surface in Hornera, unlike cinctiporids in which they are orientated within the plane of the wall. Apparent similarities in ultrastructure suggest that Hornera may provide a good model for wall growth in extinct trepostome bryozoans.     

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.     

Ultrastructure of CaCO3 deposits of terrestrial isopods (Crustacea, Oniscidea)

 The ultrastructure of the sternal CaCO₃ deposits of 3 species of the Diplochaeta and 15 of the Crinochaeta was investigated by means of scanning electron microscopy of fractured surfaces. In the Diplochaeta Li-gia italica and L. oceanica, the deposits consist exclusively of individual spherules with diameters between 0.2 and 1.4 μm. No material was observed within the spaces between the spherules. In Ligidium hypnorum, two structurally distinct regions exist. A proximal layer resembling the deposit of Ligia italica and L. oceanica and a distal layer in which the spherules appear to be fused with each other. In the species of the Crinochaeta, the CaCO₃ deposits comprise a spherular region which resembles the deposits of Ligidium hypnorum, and a homogeneous layer located between the spherular part of the deposit and the hypodermal cell layer. In some species the diameters of the spherules may be up to 3.1 μm. In the homogeneous layer and the distal spherular layer more calcium per volume can be stored than in the proximal spherular layer in which the spaces between the spherules are devoid of CaCO₃. This suggests that the multiple layered deposits are an adaptation to terrestrial life, as a consequence of the need for increased resorption of cuticular calcium. Accepted: 7 January 1997     

Ultrastructure and chemistry of soluble and polymeric lipids in cell walls from seed coats and fibres of Gossypium species

Electron-microscopic examination in conjunction with extraction procedures and chemical analysis have confirmed that a suberin-like lipid biopolymer is located within the concentric polylamellate layers found in the secondary cell walls of green cotton fibres (Gossypium hirsutum cv. green lint). A polymer of similar ultrastructure and chemical constitution also occurs mainly in the secondary seed-coat walls of the outer epidermis of both green and white varieties of G. hirsutum. The suberins composed of predominantly C₂₂ compounds are, however, markedly different from those present in the periderms of the same plants; these comprise mainly C₁₆ and C₁₈ compounds. Long-chain 1-alkanols (C₂₆–C₃₆) and alkanoic acids (C₁₆–C₃₆) are the principal components of the wax from white fibres but these lipid classes comprise a much smaller proportion of that from green fibres. unidentified highmolecular-weight compounds were the major constituents of the green-fibre was extract which also contains a number of yellow-green pigments, probably flavonoid in nature. These pigments are thought to be associated with the ultrahistochemical reaction with silver proteinate that was observed only in the green-fibre cell walls. A total of 16 wild and cultivated cotton species were examined with the electron microscope for the presence of suberin. The outer seed-coat epidermis of all the examined species but only the fibres of the wild ones were found to be suberized. Among the analysed mutants of fibre colour in G. hirsutum only the gene Lg (green lint) seemed to be associated with suberin.Abbreviations GLC gas-liquid chromatography - TLC thinlayer chromatography Fibres=fibre cells of the seed coat epidermis without fibre base; Seed coast=include the base of fibre cells, and short, so-called fuzz fibres     

Ultrastructure and Development of the Ooecial Walls in Some Calloporid Bryozoans (Gymnolaemata: Cheilostomata)

In the brood chambers (ovicells) of six calloporid cheilostomes studied each skeletal wall consists of four calcified layers: (1) a very thin superficial layer of planar spherulitic crystallites, (2) an upper (outer) layer with wall-perpendicular prismatic ultrastructure, (3) an intermediate lamellar layer, and (4) a lower (inner) wall-perpendicular prismatic layer. Comparative studies of both the ovicell wall ultrastructure and early ovicell formation showed a hypothetical opportunity for evolving complex (multilayered) skeletal walls by fusion of the initially separated gymnocystal and cryptocystal calcifications in Cheilostomata. In two species studied, a bilobate pattern in the final stage of the formation of the ooecial roof was encountered in specimens with the cuticle preserved. A possible explanation to this finding is discussed – the bilobate pattern is suggestive of the hypothetical origin of the brood chamber from (1) two flattened spines, or (2) reduction in spine number of an originally multispinous ovicell.     

Contributions of the surface of the root tip to the growth of Zea roots in soil

The development of the epidermal layer of roots of Zea is traced from the quiescent centre to the zone where root hairs develop. In the zone of cell division a three layered coat forms on the outside of the epidermal cells consisting of the outer epidermal walls, overlaid by a two-layered pellicle composed of a thick fibrillar inner layer of polysaccharide, and a thin fibrillar outer layer of protein. The epidermal cells divide several times in the same longitudinal file but rarely across a radius to give a new longitudinal file. Thus, the radial walls become much thicker than all but the original transverse walls, and packets of up to 32 daughter cells derived from a single initial may be distinguished. The pellicle develops during these divisions as a continuum over the outer walls of the daughter cells. It is proposed that the pellicle provides a stiffening to the forward end of the root which permits it to penetrate soil without bending. Support for this hypothesis is shown by the Zea mays mutant Ageotropic in which the pellicle is absent, the epidermal surface is disorganized, and which grows crookedly through soil. In the zone of extension growth of normal roots of two Zea species the pellicle thins and disappears. Circumferential strips of the pellicle were peeled off the young epidermal cells and could be stretched to twice their length. This deformation is partly the result of the pellicle stretching and breaking above the attachments of the radial walls. After normal thinning of the pellicle, detachment of the radial walls at their outer ends produces a corrugated surface in the proximal zone of the root tips. In dicotyledons (e.g., soybean), there is no similar pellicle, but a stiff root tip is produced by a long multi-layered root cap, the proximal portion of which covers the elongating epidermal surface.     

The nerves in frog skin

In the epidermis of frog skin, most nerves are situated at the top of the basal layer. More superficial nerve fibres are usually adjacent to flask cells; it is concluded that this is not a functional association, but a consequence of the pattern of moulting. There are nerve fibres in the walls of the granular glands; mucous glands appear to have no intrinsic innervation although nerves pass within a short distance of their walls. The smooth muscle bundles of the dermis are innervated, and have a physical attachment to the overlying epidermis.     

Vinealobryum guangdongensis (Pottiaceae,Musci), a new species with multicellular axillary gemmae from Guangdong,China

A new species belonging to Didymodon sensu lato, Vinealobryum guangdongensis, is described and illustrated from Nanling National Forest Park of Guangdong, China. It is characterized by noteworthily thick‐walled cells of the cauline central cylinder, ovate‐lanceolate leaves that are appressed when dry, acuminate to acute leaf apices, leaf base abruptly broadened and quickly narrowed to the insertion, leaf margins recurved in proximal 2/3 to 3/4, short‐excurrent costa with 0–1 layer of ventral stereids, laminal cells with conical or elliptical papillae either over the lumina or over transverse walls, presence of gemmae in the leaf axils, and KOH laminal color reaction red to reddish orange. This new species is compared with the most similar species and its ecology is discussed.     

ULTRASTRUCTURAL STUDY ON SPORE WALL MORPHOGENESIS IN LYCOPODIUM CLAVATUM (LYCOPODIACEAE)

Spore wall morphogenesis of Lycopodium clavatum was observed by transmission electron microscopy. The spore plasma membrane indicates the reticulate spore sculpture shortly after meiosis. The mature spore wall of this species consists of two layers, inner endospore and outer exospore. There is no perispore in the sporoderm of this species. The exospore formation begins during the tetrad stage; and this layer is divided into two distinct sublayers, an outer lamellar layer and an inner granular layer. The lamellar layer is formed on the sculptured spore plasma membrane. Additional lamellae attach to this layer in a centripetal direction. For that reason, this layer may be derived from spore cytoplasm. The granular layer is formed only in the proximal region following lamellar layer formation, and it also may be derived from spore cytoplasm. The endospore is formed lastly and seems to be derived from spore cytoplasm as well. Accordingly, the spore sculpture of this species may be under the genetic control of the spore nucleus.     

G‐fibres in storage roots of Trifolium pratense (Fabaceae): tensile stress generators for contraction

Root contraction has been described for many species within the plant kingdom for over a century, and many suggestions have been made for mechanisms behind these contractions. To move the foliage buds deeper into the soil, the proximal part of the storage root of Trifolium pratense contracts by up to 30%. Anatomical studies have shown undeformed fibres next to strongly deformed tissues. Raman imaging revealed that these fibres are chemically and structurally very similar to poplar (Populus) tension wood fibres, which are known to generate high tensile stresses and bend leaning stems or branches upright. Analogously, an almost pure cellulosic layer is laid down in the lumen of certain root fibres, on a thin lignified secondary cell wall layer. To reveal its stress generation capacities, the thick cellulosic layer, reminiscent of a gelatinous layer (G‐layer) in tension wood, was selectively removed by enzymatic treatment. A substantial change in the dimensions of the isolated wood fibre bundles was observed. This high stress relaxation indicates the presence of high tensile stress for root contraction. These findings indicate a mechanism of root contraction in T. pratense (red clover) actuated via tension wood fibres, which follows the same principle known for poplar tension wood.     

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.     

Unusual compound zooecia in the trepostome bryozoan Eostenopora from the Devonian of Guizhou,China

Enlarged, ‘compound zooecia’ are described for the first time in a trepostome bryozoan. Several of these zooecia are visible in tangential sections of Eostenopora guizhouensis (Hu) from the Devonian (Eifelian) Houershan Formation of Houershan, Dushan, southern Guizhou, China. They are broad and occupy the space of two or occasionally three or four normal autozooecia. Some have bridging walls extending partway across the enlarged zooecial chamber. Without serial sectioning, the origin of compound zooecia in E. guizhouensis is debatable. However, the existence of irregular gaps in some zooecial walls leads to the hypothesis that compound zooecia originated from the loss by resorption of the skeletal walls between two normal autozooecia. The bridging walls are interpreted as a response by the bryozoan to restore the integrity of the constituent zooecia. By analogy with the ‘Doppelgänger’ zooids of some modern cheilostome bryozoans, compound zooecia of E. guizhouensis may have housed the lophophores of more than one zooid.     

Pollen structure, tetrad cohesion and pollen-connecting threads in Pseuduvaria (Annonaceae)

The structure of the pollen of 42 species of Pseuduvaria (Annonaceae) is described. The pollen is consistently inaperturate, isopolar and radially symmetrical. Four basic patterns of exine sculpturing are identified: rugulate, verrucate, scabrate and psilate. The exine stratification of one representative species, P. macrocarpa , is shown to be entirely ectexinal. The ectexine consists of a discontinuous outer tectal layer, a columellar infratectal layer, and an inner lamellar foliated foot layer; the intine is very thin and fibrillar. The pollen is invariably released as acalymmate tetrads, in which the tectum is absent from the proximal walls. The individual pollen grains within the tetrads are connected by crosswall cohesion, involving both exine and intine; this form of cohesion has not hitherto been reported in the Annonaceae. In addition, pollen grains of neighbouring tetrads are connected in two different ways, viz. short exine connections and non-sporopollenin pollen-connecting threads. Neither of these cohesion mechanisms has previously been reported for the genus. The function of the various forms of cohesion between pollen grains and tetrads in Pseuduvaria is discussed as a mechanism to enhance the efficiency of pollination by enabling the fertilization of multiple ovules following a single pollinator visit.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 69−78.     

Pollen Organs and Seeds with Eucommiidites Pollen

The characteristic Mesozoic pollen genus Eucommiidites is described from pollen organs and seeds recovered in Cretaceous strata of North America and Europe. The pollen organs are from the lowermost Upper Cretaceous (Cenomanian) of Texas and are referred to Erdtmanitheca texensis gen. et sp. nov. They are spherical heads, composed of numerous, densely crowded, radiating pollen sacs that contain abundant well-preserved pollen. Combined LM, SEM and TEM investigations show that the pollen grains each have a distinct distal colpus flanked by two lateral colpi in an equatorial position. Pollen wall ultrastructure is gymnospermous with a thick lamellate inner layer (endexine) and an outer layer (ektexine) composed of a granular inner part and a homogeneous outer part. The endexine is thickened in the region of the colpi. Small seeds from the Lower Cretaceous (upper Berriasian to Valanginian) of Bornholm, Denmark contain abundant Eucommiidites pollen in their micropyles. The seeds are referred to Erdtmanispermum balticum gen. et sp. nov. They are ovoid, and weakly triangular in transverse section and gradually taper at the apex into an elongated tube. The megaspore membrane is granular and well developed, and apparently surrounded by three separate tissues interpreted as nucellus, a thin inner integument and a sclerified outer envelope. Eucommiidites pollen in the micropyles of the seeds has a laminated endexine and an ektexine comprising two homogeneous parts separated by a granular layer. Réévaluation of other seeds known to contain Eucommiidites pollen indicates that they share significant similarities with Erdlmanispermum and that they may have been produced by closely related plants. Comparison of “Eucommiidites plants” with other seed plants suggests that they are probably most closely related to the anthophytes comprising Bennettitales, Pentoxylales, Gnetales and angiosperms.     

Pollen wall development in Vigna vexillata I. Characterization of wall layers

Light and electron microscope observations characterized the layers that comprise Vigna vexillata L. pollen walls, and identified the timing of their development. Exine sculpturings form an unusually coarse ektexinous reticulum. The structure of the ektexine is granular; this differs from the columellate/tectate type of structure typical of most angiosperm pollen. The ektexine overlies a homogeneous-to-lamellar, electron-dense endexine, which in turn surrounds a thick, microfibrillar intine. Pollen grains are triporate and operculate, with Zwischenkörper and thickened intine underlying the apertures. The ektexine forms during the tetrad period of microspore development, the endexine and Zwischenkörper during the free microspore stage, and the intine during the bicelled (pollen) stage. Coarsely reticulate exine sculpturings and the granular structure of the patterned exine wall of the pollen grains are features that make this species suitable for detailed studies of pollen wall pattern formation.     

Suberized bundle sheaths in grasses (Poaceae) of different photosynthetic types I. Anatomy,ultrastructure and histochemistry

Summary We investigated the histochemistry and ultrastructure of the cell walls of mestome sheaths and parenchymatous bundle sheaths of ten species of grasses. The species surveyed included representatives from all the major photosynthetic types: C₃-Bromus tectorum, Phalaris arundinacea; C₄/NAD-ME-Eragrostis cilianensis, Panicum capillare; C₄/NAD-ME/PCK-Bouteloua curtipendula; C₄/PCK-Chloris gayana, Sporobolus elongatus; C₄/NADP-ME-Echinochloa crus-galli, Setaria glauca, Themeda triandra. All vein orders (designated here as major, minor and transverse) from mature leaves of each species were tested histochemically for lipids and phenols, and the majority of species were also examined with the electron microscope. A suberized lamella was detected ultrastructurally in at least some walls of major vein bundle sheath cells of all species examined. These lamellae were also present in some cells associated with the minor veins of the C₃ species and in the minor and transverse veins of the C₄/NADP-ME species. Histochemical tests for lipids and phenols consistently failed to differentiate this layer. Based on these tests, none of the vein orders in any species showed evidence of a Casparian band. In all suberized bundle sheaths, the compound middle lamella between cells with suberin lamellae is modified by the presence of phenols. These did not, however, confer resistance to acid digestion to the cell layer, in contrast to cell layers with Casparian bands. Therefore, although the mestome sheath has some features in common with the root endodermis (i.e. cells with a suberized lamella and thick, cellulosic walls which may be further modified), we could find no substantive anatomical or ultrastructural evidence for the presence of a Casparian band in any of the grass leaves investigated. The significance of these observations is discussed in the context of apoplastic permeability of these walls.