Microtubules and epithem-cell morphogenesis in hydathodes of Pilea cadierei

When cell divisions have ceased, the epithem of the hydathodes of Pilea cadierei Gagnep. et Guill. consists of small polyhedral cells exhibiting a meristematic appearance, and completely lacks intercellular spaces. The cortical microtubules in epithem cells exhibit a unique organization: they are not scattered along the whole wall surface but form groups lying at some distance from each other. In sections, from two to eight groups of microtubules can be observed, each lining a wall region averaging between 0.5 and 1.5 m in length. These groups represent sections of microtubule bundles girdling a major part or the whole of the cell periphery. They are connected to one another by anastomoses, forming a microtubular reticulum. The assembly of microtubule bundles is followed by the appearance of distinct local thickenings in the adjacent wall areas. The cellulose microfibrils in the thickenings are deposited in parallel to the underlying microtubules. Gradually, the vacuolating epithem cells undergo swelling, except for the areas bounded by the wall thickenings. Since the latter, and actually their constituent bundles of cellulose microfibrils, cannot extend in length the differential cell growth results in schizogenous formation of intercellular spaces between contiguous cell walls at their thickened regions. The spaces then broaden and merge to become an extensive intercellular space system. As a result of the above processes, the epithem cells become constricted and finally deeply lobed. The observations show that (i) the cortical microtubules are intimately involved in the morphogenesis of the epithem cells and (ii) the initiation and development of the epithem intercellular spaces is a phenomenon directly related to cell morphogenesis and therefore to the cortical microtubule cytoskeleton. The sites of initiation of these spaces are highly predictable.     

Structures and roles of the polymorphic forms of tobacco mosaic virus protein. 8. Initial stages of assembly of nucleoprotein rods from virus RNA and the protein disks

The initial stages of the assembly of tobacco mosaic virus have been investigated by reassembling the RNA with a radioactively labelled protein disk preparation and then completing the reaction by the addition of a large excess of an unlabelled disk preparation. This gives measurements of the numbers of subunits incorporated at early times and the growth curves have been plotted.These curves have been analysed in terms of a bimolecular nucleation reaction, which is first order in the disk concentration, with a rate constant of 1.3 × 10³ mol^?1 s^?1, and then an elongation which saturates at high protein concentrations to a maximum rate of 7.6 subunits s^?1, with a K_m of 0.84 mg/ml for the disk preparation.These kinetic parameters, and the predicted overall assembly curves, have been compared with data previously determined by other methods and agree closely, showing that the different experimental techniques give consistent results. The measurements are fully compatible with our earlier hypotheses Butler &; Klug 1971 that the nucleation with virus RNA and protein disks is rapid compared with the subsequent rod elongation and that this elongation can occur most rapidly directly from the protein disks. They are not compatible with the contention of some other workers that elongation cannot occur directly from disks, but only from the smaller A-protein.     

Leaf teeth, transpiration and the retrieval of apoplastic solutes in balsam poplar

The rapid flow of the transpiration stream through major veins to leaf teeth was followed in leaves of Populus balsamifera L., using the tracer sulphorhodamine G (SR), which probes for cells with H⁺-extrusion pumps. The tracer accumulated quickly in the hydathodes of the teeth. It was shown by freeze-substitution and anhydrous processing that SR was taken up by phloem parenchyma and epithem cells of the hydathode. When ¹⁴C-labelled aspartate was fed to the leaves in the transpiration stream, it also was taken up most strongly by the same phloem parenchyma and epithem cells. It is proposed that one function of the hydathodes in leaf teeth is the retrieval of solutes from the transpiration stream.     

Studies on the Epidermal Structure of Tingia carbonica

Two leafy specimens of Tingia carbonica were collected as impression from Shanxi formation of Permian, from Inner Mongolia, China. The epidermal structure of its leaves is reported in this paper. Under SEM, well preserved epidermal cells as well as some concaves on the surface of large leaves were clearly recognized. The epidermal cells are approximately rectangular in shape, about 100～150 um long and 20～30 um wide. They are arranged longitudinally parallel to veins. The concaves usually in rows are round or elliptical, about 0.65～0.35 mm long and 0.2～0.35 mm wide. Density of concaves is about 1.8/ mm² and no stomata occur inside the concaves. In all probability, this is the upper epidermis. On the other side of the epidermis, anomocytic stomata are scattered irregularly, each with 5～6 epidermal cells around. The stomatal apertures are about 35.8 µm long, and 18.7 um wide, which is organised parallel to the common epidermal cells. As far as shape and size is concerned, it is similar to that described on the upper epidermis. Density of the stomata is about 60/mm². In all Probability, this is the lower epidermis. The ecological preference and classification of Tingia are discussed according to these new characters of the epidermis andsubordinate struture of the leaf.     

Intercellular junctions during development and in tissue cultures ofDrosophila melanogaster: An electron-microscopic study

Summary The present investigation analyzes intercellular junctions in tissues with different developmental capacities. The distribution of junctions was studied inDrosophila embryos, in imaginal disks, and in cultures of disk cells that were no longer able to differentiate any specific pattern of the adult epidermis.The first junctions —primitive desmosomes andclose membrane appositions — already appear in blastoderm.Gap junctions are first detected in early gastrulae and later become more and more frequent.Zonulae adhaerentes are formed around 6 h after fertilization, whileseptate junctions appear in the ectoderm of 10-h-old embryos.Inwing disks of all stages studied (22–120 h), three types of junctions are found: zonulae adhaereentes, gap junctions, and septate junctions. Gap junctions, which are rare and small at 22 h, increase in number and size during larval development. The other types of junctions are found between all cells of a wing disk throughout development.All types of junctions that are found in normal wing disks are also present in theimaginal disk tissues cultured in vivo for some 15 years and in thevesicles of imaginal disk cells grown in embryonic primary cultures in vitro. However, gap junctions are smaller and in the vesicles less frequent than in wing disks of mature larvae.Thus gap junctions, which allow small molecules to pass between the cells they connect, are present in the early embryo, when the first developmental decisions take place, and in all imaginal disk tissues studied, irrespective of whether or not these are capable of forming normal patterns.     

Leaf anatomy and ultrastructure of the Crassulacean-acid-metabolism plant Kalanchoe daigremontiana

Light-microscopic analysis of leaf clearings of the obligate Crassulacean-acid-metabolism (CAM) species Kalanchoe daigremontiana Hamet et Perr. has shown the existence of unusual and highly irregular venation patterns. Fifth-order veins exhibit a three-dimensional random orientation with respect to the mesophyll. Minor veins were often observed crossing over or under each other and over and under major veins in the mesophyll. Paraffin sections of mature leaves show tannin cells scattered throughout the mesophyll rather evenly spaced, and a distinct layer of tannin cells below the abaxial epidermis. Scanning electron microscopy showed that bundle-sheath cells are distinct from the surrounding mesophyll in veins of all orders. Transmission electron microscopy demonstrated developing sieve-tube elements in expanded leaves. Cytosolic vesicles produced by dictyosomes undergo a diurnal variation in number and were often observed in association with the chloroplasts. These vesicles are an interesting feature of cell ultrastructure of CAM cells and may serve a regulatory role in the diurnal malic-acid fluctuations in this species.Abbreviations CAM Crassulacean acid metabolism - SEM scanning-electron microscopy - TEM transmission-electron microscopy     

Disk formation in retinal cones of Tupaia belangeri (Scandentia)

Existing hypotheses on the mode of disk formation in the photoreceptor cells of mammals appear to be incompatible: (1) plasma membranes of adjacent evaginations form a disk which, subsequently, is internalized by a disk rim; (2) pinocytotic vesicles are pinched off from the plasma membrane and fuse into a larger vesicle, which flattens and forms a disk. We have studied the development of the cone outer segment and the disk formation in Tupaia belangeri by transmission electron microscopy. During the first two postnatal weeks, the distal part of the single cilium, which is inserted apically on the inner segment, becomes balloon-shaped. Apical to the axoneme, it contains tubular and vesicular material, which, most probably, has been detached from the axonemal microtubules. These tubules and vesicles do not contribute to disks. The balloon-shaped expansion, later retained as the ciliary backbone, establishes the contact with the pigment epithelium. Formation of disks, from the 12-day-old Tupaia onwards, occurs between adjacent evaginations at the outer segment base. The initial disk rims are “hooked” to the ciliary axonemal microtubules. The axonemal microtubules are involved in the initiation and in the alignment of the disks. Disk rim formation and, thus, internalization of disks proceeds from the base to the apex of the outer segment, that is, from the younger to the older disks. In the adult Tupaia, an uneven progression of disk rim formation on both sides of the axoneme is found among consecutive disks. The seemingly incompatible hypotheses on the mode of disk formation reflect a heterochrony of the internalization of membranes and of the disk formation among different mammals and, possibly, between cones and rods. Received: 24 July 1997 / Accepted: 10 September 1997     

Integument and epidermal sensory structures of Myzostoma cirriferum (Myzostomida)

Summary The fine structure of the integument of Myzostoma cirriferum is described with special attention to the integument sensory areas. Hypotheses about the function and a functional model of these are proposed. The integument consists of an external pseudostratified epithelium with cuticle (the epidermis) covering a parenchymo-muscular layer (the dermis). The dermis includes two types of cells: muscular fibers of the double obliquely striated type and parenchymal cells. Differences occur in the epidermis, which consists either of a large non-innervated myoepithelial area (viz. the regular epidermis). or of several rather localized sensory-secretory areas associated with discrete nerve proceses (viz. the sensory epidermis). The regular epidermis is made up of three types of cell: covering cells, ciliated cells and myoepithelial cells. The sensory epidermis shows small or marked structural variations from the regular epidermis. Small variations occur in the cirri, the buccal papilla, the body margin, the parapodia and the parapodial folds where nerve processes insinuate between epidermal cells. They are thought to be mechanoreceptor sites that could give information on the structural variations of the host's integument and participate in the recognition of individuals of the same species. The sensory epidermis differs markedly from the regular eidermis in the four pairs of lateral organs. Each lateral organ consists of a villous and ciliated dome-like central part, surrounded by a peripheral fold. The epidermis of the fold's inner part (viz. the part facing the central dome) is made up of secretory cells, while that of the fold's outer part is similar to the regular epidermis. The epidermis of the dome includes vacuolar cells, sensory cells and a different type of secretory cell. Lateral organs are presumed to be both chemoreceptors and mechanoreceptors. They could allow the myzostomids to recognize the host's integument and prevent them from shifting on the surrounding inhospitable substrate.     

ONTOGENY OF THE TENDRIL IN VITIS VINIFERA

The apical meristem of the grape tendril possesses several remarkable features: bilateral symmetry, a minimal number of appendages, and an exceedingly brief period of apical meristem activity. The cellular configuration of the apex changes from tunica-corpus to zonate, as rudimentary leaves and branch-tendril apices are initiated. Eventually the apical meristem of the tendril itself ceases meristematic activity and differentiates as a large hydathode. Typical spongy epithem tissue, copious xylary tissue, and water-pores in the epidermis characterize the hydathode. Numerous vascular strands traverse the length of the tendril and terminate in enlarged tracheary elements adjacent to the epithem. Cessation of meristematic activity follows lowered mitotic rate in the summit and accelerated differentiation below and within the meristem.     

Marginal and laminar hydathode-like structures in the leaves of the desiccation-tolerant angiosperm Myrothamnus flabellifolius Welw.

The fan-shaped leaves of the resurrection plant Myrothamnus flabellifolius Welw. fold during episodes of drought and consequent desiccation of the tissue. The leaf teeth of M. flabellifolius have several features characteristic of hydathodes. Tracheary elements from the three vein endings that converge in each leaf tooth subtend and extend into a cluster of cells significantly smaller than those of the adjacent mesophyll. The stomata overlying this putative epithem are larger than the other stomata on the leaf surface. Crystal violet is absorbed via these stomata in non-transpiring leaves, suggesting that they are water pores. Two to four such water pores occur per hydathode and are readily distinguished in desiccated leaves. Laminar hydathodes apparently also occur in the leaves of M. flabellifolius. Branched vein endings that terminate in short, wide tracheary elements subtend the outer edges of the abaxial leaf ridge, which otherwise lack stomata, and coincide with regions of crystal violet uptake. Guttation could not be induced in M. flabellifolius. However, desiccated leaves readily absorb liquid water through the leaf surface. The use of Calcafluor White to trace the pathway of apoplastic water movement suggests a role for both types of hydathode in foliar water uptake during rehydration while the accumulation of Sulphorhodamine G (indicating solute retrieval from the apoplast) in the epithem of transpiring plants suggests the hydathodes may be a pathway of water loss in the desiccating leaf.     

Hormonal Induction of Vascular Differentiation in Cultured Zinnia Leaf Disks

Mesophyll cells differentiated into tracheary elements whenZinnia leaf disks were cultured in media containing sufficientauxin and cytokinin. Moderate increases in the levels of phytohormonesinduced the recruitment of adjacent cells into the preexistingvasculature, resulting in the "expansion" of leaf veins. Higherhormonal concentrations induced unpatterned differentiationinto tracheary elements throughout the mesophyll and epidermis.This novel system should facilitate the study of organized vasculardifferentiation. (Received July 30, 1988; Accepted October 31, 1988)     

Anatomically preserved leaves of the conifer Notophytum krauselii (Podocarpaceae) from the Triassic of Antarctica

Permineralized leaves of the Triassic podocarpaceous conifer Notophytum krauselii are described from the Fremouw Formation of Antarctica. The leaves are elongate and apetiolate with 8-12 parallel veins. The adaxial epidermis consists of rows of rectangular to pentagonal cells; the abaxial epidermis is papillate. Longitudinally oriented stomata occur on both surfaces. An adaxial palisade layer is present and auxiliary sclereids are common in the mesophyll. The vascular bundles have a weakly defined sheath and are flanked by transfusion tracheids. Bundles in the basipetal area of the leaf are capped by sclerotic tissue and subtended by resin canals. These leaves are superficially similar to those of the extant podocarp genus Nageia, but probably represent a distinct acquisition of this leaf type within the Podocarpaceae. Notophytum leaves are similar to the common compression fossil Heidiphyllum elongatum and may be closely related or even conspecific. Evidence from Antarctica suggests that Heidiphyllum and the seed cone Telemachus were produced by the same plant, and may be closely related to several other early Mesozoic conifers with multiveined leaves.     

Distribution of Lithocysts, Trichomes, Hydathodes and Stomata in Leaves of Pilea cadierei Gagnep. & Guill. (Urticaceae)

In leaves of Pilea cadierei lithocysts, trichomes and hydathodesin the upper epidermis, and lithocysts, trichomes and anomocyticstomata in the lower epidermis are components of a regular distributionpattern in which spacing probably depends upon inhibitory interactionsbetween initial cells during leaf development. Anisocytic stomata,which also occur in the lower epidermis, show a clumped distributionpattern that results from their relatively late formation aftercell division in the epidermis is otherwise complete, and fromthe inhibitory influence of established lithocysts, trichomesand anomocytic stomata. Ordered spacing of anisocytic stomatawithin groups probably results from mutual inhibition betweeninitial cells at the time of inception. Lithocysts associatedwith primary and secondary veins are elongated along the veinaxis. Elsewhere in the leaf lithocyst, orientation is obliquerelative to the midrib and shows a correlation with vein orientationaround the time of lithocysts elongation. Lithocyst, cystolith, trichome, hydathode, stomata, Pilea cadierei     

Control of pupal commitment in the imaginal disks of Precis coenia (Lepidoptera: Nymphalidae)

When final (5th) instar larvae of Precis coenia were treated with the juvenile hormone analog (JHA) methoprene, they underwent a supernumerary larval molt, except for certain regions of their imaginal disks, which deposited a normal pupal cuticle. Evidently those regions had already become irreversibly committed to pupal development at the time JHA was applied. By applying JHA at successively later times in the instar, the progression of pupal commitment could be studied. Pupal commitment in the proboscis, antenna, eye, leg and wing imaginal disks occurred in disk-specific patterns. In each imaginal disk there were distinct initiation sites where pupal commitment began during the first few hours of the final larval instar, and from which commitment spread across the remainder of the disk over a 2- to 3-day period. The initiation sites were not always located in homologous regions of the various disks. As a rule, pupal commitment also spread from imaginal disk tissue to surrounding epidermal tissue. The regions of pupal commitment in all disks except those of the wings, coincided with the regions of growth of the disk. Only portions of the disk that had undergone cell division and growth underwent pupal commitment. Shortening the growth period did not prevent pupal commitment in the wing imaginal disk, indicating that, in this disk at least, a normal number of cell divisions was not crucial in reprogramming of disk cells for pupal cuticle synthesis. The apparent growth spurt of imaginal disks that occurs during the last part of the final larval instar is merely the final stage of normal and constant exponential growth. Juvenile hormone (JH) and ecdysteroids appeared to play little role in the regulation of normal imaginal disk growth. Instead, growth of the disks may be under intrinsic control. Interestingly, even though endogenous fluctuation in JH titers do not affect imaginal disk growth, exogenous JHA proved able to inhibit both pupal commitment, cell movement, and growth of the disks during the last larval instar. This function of JH could be important under certain adverse conditions, such as when metamorphosis is delayed in favor of a supernumerary larval molt.     

Clonal analysis of the undifferentiated wing disk of Drosophila

Using a new cell marker, we have examined the early clonal restrictions in wing imaginal disks from late third instar larvae of Drosophila. Large clones do not significantly alter the gross structure of the disks, allowing us to map the clones relative to morphological landmarks. Clones in the posterior region of the disks behave in a similar way to clones in the adult cuticle; that is, they appear to be restricted to a defined compartment, and the presumptive anterior/posterior compartment border defined by these clones is located in a similar place in every disk. In contrast, clones in the anterior region of the wing disks often cross into the region normally occupied by the posterior compartment and, especially near the margins of the disk, show no common posterior boundary. We believe that the anterior clones are “pushing” the anterior/posterior compartment border, and that this pushing is related to the growth advantage of the marked cells, which are Minute⁺ in a Minute background. Finally, we find that clones do not cross between the adepithelial cells, which contribute to the adult musculature, and the disk epithelium.     

Is the adhesive material secreted by sea urchin tube feet species‐specific?

Sea urchin adoral tube feet are highly specialized organs that have evolved to provide efficient attachment to the substratum. They consist of a disk and a stem that together form a functional unit. Tube foot disk tenacity (adhesive force per unit area) and stem mechanical properties (e.g., stiffness) vary between species but are apparently not correlated with sea urchin taxa or habitats. Moreover, ultrastructural studies of sea urchin disk epidermis pointed out differences in the internal organization of the adhesive secretory granules among species. This prompted us to look for interspecific variability in the composition of echinoid adhesive secretions, which could explain the observed variability in adhesive granule ultrastructure and disk tenacity. Antisera raised against the footprint material of Sphaerechinus granularis (S. granularis) were first used to locate the origin of adhesive footprint constituents in tube feet by taking advantage of the polyclonal character of the generated antibodies. Immunohistochemical assays showed that the antibodies specifically labeled the adhesive secretory cells of the disk epidermis in the tube feet of S. granularis. The antibodies were then used on tube foot histological sections from seven other sea urchin species to shed some light on the variability of their adhesive substances by looking for antibody cross‐reactivity. Surprisingly, no labeling was observed in any of the species tested. These results indicate that unlike the adhesive secretions of asteroids, those of echinoids do not share common epitopes on their constituents and thus would be “species‐specific.” In sea urchins, variations in the composition of adhesive secretions could therefore explain interspecific differences in disk tenacity and in adhesive granule ultrastructure. J. Morphol., 2011. © 2011 Wiley Periodicals, Inc.     

Fossil leaves of Populus from the Middle Miocene of Yunnan,SW China

Today southern Yunnan, SW China, has a tropical or subtropical climate and seasonal rainforests. In the past, some temperate elements were also present. In this paper, a new species of Populus is reported from the Middle Miocene deposits in Zhenyuan. Its leaves are ovate or ovate-suborbicular, with serrate margins. They have a shallowly cordate to cuneate base without glands, short acuminate apex, and salicoid teeth with spherical glands. The veins are glabrous but unicellular hair bases occur on the lower epidermis of the lamina. Stomata are confined to the lower epidermis. The presence of Populus in the Middle Miocene of the region indicates an expansion of the genus into low-latitude Asia in the late Cenozoic and a more complicated history of vegetational change in southern Yunnan than has so far been assumed.     

Differential membrane protein phosphorylation in bovine retinal rod outer segment disk membranes as a function of disk age

The outer segment portion of photoreceptor rod cells is composed of a stacked array of disk membranes. Newly formed disks are found at the base of the rod outer segment (ROS) and are relatively high in membrane cholesterol. Older disks are found at the apical tip of the ROS and are low in membrane cholesterol. Disk membranes were separated based on their membrane cholesterol content and the extent of membrane protein phosphorylation determined. Light induced phosphorylation of ROS disk membrane proteins was investigated using magic angle spinning³¹P NMR. When intact rod outer segment preparations were stimulated by light, in the presence of endogenously available kinases, membrane proteins located in disks at the base of the ROS were more heavily phosphorylated than those at the tip. SDS-gel electrophoresis of the phosphorylated disk membranes subpopulations identified a phosphoprotein species with a molecular weight of approximately 68–72 kDa that was more heavily phosphorylated in newly formed disks than in old disks. The identity of this phosphoprotein is presently under investigation. When the phosphorylation reaction was carried out in isolated disk membrane preparations with exogenously added co-factors and kinases, there was no preferential protein phosphorylation. Taken collectively, these results suggest that within the ROS there is a protein phosphorylation gradient that maybe indicative of co-factor or kinase heterogeneity.     

Mobile and immobile endoplasmic reticulum in onion bulb epidermis cells: short- and long-term observations with a confocal laser scanning microscope

The endoplasmic reticulum (ER) of onion bulb scale epidermis cells consists of long, tubular strands lying deep in the cytoplasm which move quickly and a less mobile peripheral network of tubules and cisternae that change in position, shape and size but that also have immobile, fixed, sites (IFSs). IFSs occur in junctions, at vertexes and at blind endings of tubules as well as at the edges and the surface of cisternae. They are regularly arranged in helicoidal rows and may be knot- or ring-like in structure. They become enlarged by treatment with oryzalin but not with colchicine. They persist for long times (for more than 30 min); together with pulling forces, the surface tension and other factors, they determine the configuration and motion of the peripheral network. New polygons of the network are mainly formed by the development of new tubules that become joined with other parts of the network. Polygons disappear by contraction and fusion of tubules. The inner, rapidly moving ER tubules remain connected with the peripheral network over longer distances by sliding junctions. Cytochalasin D causes an accumulation of the ER into patches, a fusion of tubules into cisternae and changes in shape, which indicate the loss of pulling forces. In contrast to animal cells (but like the movement of the inner tubular strands), the latter is dependent upon the actomyosin system; microtubules are not involved. Despite the differences in the organizing components, the peripheral ER in onion bulb scale epidermis cells and that of the borders of cultured animal cells are similar in morphology and motility.     

STUDIES ON THE LEAF OF AMARANTHUS RETROFLEXUS (AMARANTHACEAE***): MORPHOLOGY AND ANATOMY

The leaf of Amaranthus retroflexus L. was examined with the light microscope to determine its vasculature and the spatial relationship of the vascular bundles to the mesophyll. Seven leaf traces enter the petiole at the node and form an arc that continues acropetally in the petiole as an anastomosing system of vascular bundles. Upon entering the lamina, the arc of bundles gradually closes and forms a ring of anastomosing bundles that constitutes the primary vein, or midvein, of the leaf. As the midvein progresses acropetally, branches of the bundles nearest the lamina diverge outward and continue as secondary veins toward the margin on either side of the lamina. Along its course the midvein undergoes a gradual reduction in number of bundles until only one remains as it approaches the leaf tip. Tertiary veins arise from the secondaries, and minor veins commonly arise from all orders of major veins, as well as from other minor veins. All of the major veins are associated with rib tissue, although the ends of the tertiaries may resemble minor veins, which are completely encircled by chlorenchymatic bundle sheaths and mesophyll cells that radiate out from the sheaths. A specialized minor vein, the fimbrial vein, occurs just inside the margin of the leaf. Most of the mesophyll cells—the so-called “Kranz mesophyll cells”—are in direct contact with the bundle sheaths, but some—the so-called “nonKranz mesophyll cells”—lack such contact. Non-Kranz mesophyll cells are especially prominent where they form a network of mostly horizontally oriented cells just above the lower epidermis. Guard cells of both the upper and lower epidermis are spatially associated with nonKranz mesophyll cells.