Specialized Parenchyma Cells Research Articles

Changes in structure and dimension of plasmodesmata in the phloem loading pathway in Tecoma capensis (Bignoniaceae) - locating the polymer trap

Tecoma capensis is one of relatively few dicotyledonous plants in which the phloem in the veins of leaves, contain specialized parenchyma cells (called intermediary cells) in addition to sieve tubes and companion cells. These plants convert sucrose to the raffinose family oligosaccharides (RFOs), which form the primary transport sugar. The fact that the interface between bundle sheath and intermediary cells in RFO-transporting species always contain high numbers of plasmodesmata (Pd) in the BS-IC interface raises questions on the mechanism driving RFO accumulation and the subsequent loading of the RFOs into the phloem. Our primary focus was thus to make a detailed study of the plasmodesmata in the phloem loading pathway in Tecoma to determine (a) plasmodesmatal frequencies at all cell-to-cell interfaces in the phloem loading pathway and (b) to undertake a comprehensive study of the ultrastructure of plasmodesmata in the loading pathway. We noted that the average plasmodesmatal total diameter decreased from 49.5 nm in the common walls between mesophyll cells, to 47.5 nm between mesophyll and bundle sheath cells, to 31.2 nm at the interface between bundle sheath and intermediary cells and was reduced further to 9.7 nm in the IC. From our observations the radius of the cytoplasmic sleeve (through which intercellular transport takes place), was condensed to approximately 1.5 nm across, and there are indications that the plasma membrane and/ or the desmotubule may be lost within these PD.

Resin flow in conifers

Resins are plant exudates of economic importance used by plants as defence. They flow out of resin ducts, open and long tube-like intercellular spaces lined by a layer of specialized parenchyma cells, called the epithelium, which secrete resin into the duct lumen. A model that describes resin flow in conifers is presented to investigate how duct structure, resin loading, crystallisation, and viscosity affect flow and could explain differences between species. Considering resin viscosity, the structure of resin ducts, and a pressure-driven resin loading through the duct wall, the unsteady Stokes equation was applied. There is an increase in flow towards the open end that is favoured by the duct geometry. Both flow and pressure depend on the loading mechanism and on the duct resistance, which depends on the duct geometry, viscosity and duct wall permeability to resin. These results confirm previous measurements and observations made on Pinaceae and seem to be physiologically advantageous for the defence role of resin. Understanding of how these physiological and morphological parameters affect resin flow might be useful for selecting varieties and species having a high resin yielding capacity. The model presented in this paper is also applicable to other external secretory systems in plants.

Effects of de- and rehydration on food-conducting cells in the moss Polytrichum formosum: a cytological study.

Moss food-conducting cells (leptoids and specialized parenchyma cells) have a highly distinctive cytology characterized by a polarized cytoplasmic organization and longitudinal alignment of plastids, mitochondria, endoplasmic reticulum and vesicles along endoplasmic microtubules. Previous studies on the desiccation biology of mosses have focused almost exclusively on photosynthetic tissues; the effects of desiccation on food-conducting cells are unknown. Reported here is a cytological study of the effects of de- and rehydration on food-conducting cells in the desiccation-tolerant moss Polytrichum formosum aimed at exploring whether the remarkable subcellular organization of these cells is related to the ability of mosses to survive desiccation. Shoots of Polytrichum formosum were dehydrated under natural conditions and prepared for transmission and scanning electron microscopy using both standard and anhydrous chemical fixation protocols. Replicate samples were then fixed at intervals over a 24-h period following rehydration in either water or in a 10 microM solution of the microtubule-disrupting drug oryzalin. Desiccation causes dramatic changes; the endoplasmic microtubules disappear; the nucleus, mitochondria and plastids become rounded and the longitudinal alignment of the organelles is lost, though cytoplasmic polarity is in part retained. Prominent stacks of endoplasmic reticulum, typical of the hydrated condition, are replaced with membranous tubules arranged at right angles to the main cellular axis. The internal cytoplasm becomes filled with small vacuoles and the plasmalemma forms labyrinthine tubular extensions outlining newly deposited ingrowths of cell wall material. Whereas plasmodesmata in meristematic cells at the shoot apex and in stem parenchyma cells appear to be unaffected by dehydration, those in leptoids become plugged with electron-opaque material. Starch deposits in parenchyma cells adjoining leptoids are depleted in desiccated plants. Rehydration sees complete reestablishment over a 12- to 24-h period of the cytology seen in the control plants. Oryzalin effectively prevents leptoid recovery. The results point to a key role of the microtubular cytoskeleton in the rapid re-establishment of the elaborate cytoplasmic architecture of leptoids during rehydration. The reassembly of the endoplasmic microtubule system appears to dictate the time frame for the recovery process. The failure of leptoids to recover normal cytology in the presence of oryzalin further underlines the key role of the microtubules in the control of leptoid cytological organization.

Conducting tissues and phyletic relationships of bryophytes.

Internal specialized conducting tissues, if present, are restricted to the gametophytic generation in liverworts while they may occur in both generations in mosses. Conducting tissues are unknown in the anthocerotes. Water-conducting cells (WCCs) with walls perforated by plasmodesma-derived pores occur in the Calobryales and Pallaviciniaceae (Metzgeriales among liverworts and in Takakia among mosses. Imperforate WCCs (hydroids) are present in bryoid mosses. A polarized cytoplasmic organization and a distinctive axial system of microtubules is present in the highly specialized food-conducting cells of polytrichaceous mosses (leptoids) and in less specialized parenchyma cells of the leafy stem and seta in other mosses including Sphagnumn. A similar organization, suggested to reflect specialization in long-distance symplasmic transport of nutrients, also occurs in other parts of the plant in mosses, including rhizoids and caulonemata, and may be observed in thallus parenchyma cells of liverworts. Perforate WCCs in the Calobryales, Metzgeriales and Takakia, and hydroids in bryoid mosses, probably evolved independently Because of fundamental differences in developmental design, homology of any of these cells with tracheids is highly unlikely. Likewise, putative food-conducting of bryophytes present highly distinctive characteristics and cannot be considered homologous with the sieve cells of tracheophytes.

Regeneration of tissue following cavity formation in the vascular cylinders of Pisum sativum (Fabaceae) primary roots

The reorganization of vascular cylinders of pea (Pisum sativum, cv. Alaska) primary roots following the formation of vascular cavities was examined by light and electron microscopy. Cavities usually began forming ~20 mm from the root tip and were continuous to ~90 mm from the tips in roots 150 mm long, where they began filling with specialized parenchyma cells (SP cells). SP cells were usually produced by enlargement of parenchymous cells of the primary xylem at cavity margins. Depending on the extent and shape of the cavity, they were also sometimes produced by primary phloem parenchyma and early derivatives of the vascular cambium. Enlargement and some divisions of SP cells continued until a cavity was completely filled by them. SP cells proceeded through a series of cytoplasmic changes as they developed. First the cytoplasmic layer became thicker and more electron dense than surrounding cells. As SP cells enlarged there was an increase in vesicular traffic and the cytoplasm became less electron dense. Ultimately the cytoplasm thinned further, organelles degenerated, and the tonoplast sometimes broke down. SP cells did not form secondary walls. X-ray microanalysis revealed that SP cells accumulated potassium and rubidium to the same degree as cortical and xylem parenchyma cells and to a greater degree than immature secondary and late-maturing tracheary elements.

The Occurrence of Vascular Cavities and Specialized Parenchyma Cells in the Roots of Cool‐season Legumes

The seeds of 20 legume species were grown in the greenhouse or in growth chambers at different temperatures. Under warm temperature conditions (above 15 °C), six species, pea (Pisum sativum), broadbean (Vicia faba), chickpea (Cicer arietinum), lentil (Lens culinaris), wild lupine (Lupinus latifolius), and soybean (Glycine max), formed cavities in the vascular cylinder of their primary roots, which in turn became filled by the ingrowth of specialized parenchyma cells (SP cells). When these species were grown at low temperature (below 15 °C), however, a “normal” vascular cylinder formed in the primary roots with late-maturing metaxylem vessel members differentiating in the center. These species were all cool-season legumes except soybean, a warm-season legume, which sometimes also formed cavities and SP cell ingrowths. The occurrence of cavities and SP cells therefore was restricted to the cool-season legumes (except soybean) when they were grown under warm temperature conditions. The position and size of cavities varied among these species. Pea and broadbean usually formed large, axially elongated cavities in the central vascular cylinder, or in the xylem poles. Others formed smaller cavities of various lengths.

Temperature-Induced Cavities and Specialized Parenchyma Cells in the Vascular Cylinder of Pea Roots

Pea seeds (Pisum sativum L.) of six cultivars were planted in the field, in the greenhouse, or in growth chambers, in five different media, in light or dark, and at various temperatures (10–32 C). Under all conditions above 15 C the central portion of the vascular cylinder, in all cultivars except “Ageotropum,” tended to form cavities in almost every primary root examined. These cavities then became filled by the ingrowth of specialized parenchyma cells (SP cells). The formation of cavities and SP cells was temperature dependent since the roots grown below 15 C always formed central metaxylem tracheary elements (MTEs), without cavities and SP cells. Cavities and SP cells did not form over the entire root length. When the roots were longer than 3 cm, they started to form cavities and SP cells and continued for an additional 10–30 cm. After that, late MTEs formed in the central vascular cylinder, and no cavities and SP cells occurred regardless of temperature. Within an individual root grown above 15 C, cavities and SP cells tended to form during periods of fast growth, while during periods of slow growth large central MTEs formed instead.

TEMPERATURE‐INDUCED CAVITIES AND SPECIALIZED PARENCHYMA CELLS IN THE VASCULAR CYLINDER OF PEA ROOTS

Pea seeds (Pisum sativum L.) of six cultivars were planted in the field, in the greenhouse, or in growth chambers, in five different media, in light or dark, and at various temperatures (10–32 C). Under all conditions above 15 C the central portion of the vascular cylinder, in all cultivars except “Ageotropum,” tended to form cavities in almost every primary root examined. These cavities then became filled by the ingrowth of specialized parenchyma cells (SP cells). The formation of cavities and SP cells was temperature dependent since the roots grown below 15 C always formed central metaxylem tracheary elements (MTEs), without cavities and SP cells. Cavities and SP cells did not form over the entire root length. When the roots were longer than 3 cm, they started to form cavities and SP cells and continued for an additional 10–30 cm. After that, late MTEs formed in the central vascular cylinder, and no cavities and SP cells occurred regardless of temperature. Within an individual root grown above 15 C, cavities and SP cells tended to form during periods of fast growth, while during periods of slow growth large central MTEs formed instead.

Unusual Mitochondrial Aggregates in Companion Cells ofAristolochia ClusiiLojacono

SUMMARYIn leaf «companion cells» (specialized parenchyma cells associated with sieve-tube elements) of Aristolochia clusii Lojacono, unusual clusters of conspicuous mitochondria regularly and intimately associated are observed. These clusters, which exhibit a striking similarity to those observed in rod cells of the cat retina, are correlated, in Aristolochia, with «wein-loading» and sucrose translocation into the sieve tubes.

Observations on the structure and osmotic potentials of parenchyma associated with the internal phloem of potato tubers

The structure and osmotic potentials of parenchyma cells associated with internal phloem of growing, dormant, and sprouting potato tubers (Solanum tuberosum L. cv. Irish Cobbler) were studied. Companion cells, specialized parenchyma cells, and storage parenchyma cells were recognizable by differences in structural characteristics. Companion cells were usually densely cytoplasmic with numerous large mitochondria and plastids with little or no starch. The specialized parenchyma cells which surrounded groups of sieve tubes and companion cells had numerous vacuoles of various size, small mitochondria, plastids with very little or no starch, and protein inclusions. Storage parenchyma cells had amyloplasts with very large starch grains, a large central vacuole with osmiophilic inclusions, and few organelles in a peripheral cytoplasm. The osmotic potentials of specialized parenchyma cells and storage parenchyma cells were similar and did not vary regardless of whether the tubers were growing, dormant, or sprouting.

Distribution of Photosynthetic Enzymes between Mesophyll, Specialized Parenchyma and Bundle Sheath Cells of Arundinella hirta

Arundinella hirta L. is a C(4) plant having an unusual C(4) leaf anatomy. Besides mesophyll and bundle sheath cells, A. hirta leaves have specialized parenchyma cells which look morphologically like bundle sheath cells but which lack vascular connections and are located between veins, running parallel to them. Activities of phosphoenolpyruvate and ribulose-1,5-bisphosphate carboxylases and phosphoenolpyruvate carboxykinase, NADP-and NAD-malic enzymes were determined for whole leaf extracts and isolated mesophyll protoplasts, specialized parenchyma cells, and bundle sheath cells. The data indicate that A. hirta is a NADP-malic enzyme type C(4) species. In addition, specialized parenchyma cells and bundle sheath cells are enzymatically alike. Compartmentation of enzymes followed the C(4) pattern with phosphoenolpyruvate carboxylase being restricted to mesophyll cells while ribulose-1,5-bisphosphate carboxylase and decarboxylating enzymes were restricted to bundle sheath and specialized parenchyma cells.

REACTIONS OF ALKALOID AND HISTOCHEMICAL INDICATORS IN LATICIFERS AND SPECIALIZED PARENCHYMA CELLS OF CATHARANTHUS ROSEUS (APOCYNACEAE)

Chromic acid, iodine‐potassium iodide and Dragendorff reagent were employed to identify reactive cells that may indicate sites of alkaloid accumulation in fresh tissues and latex of C. roseus. Laticifers in all parts of the mature plant and certain parenchyma cells in the cortex and pith regions accumulated reaction products of these alkaloid indicators. These same cells showed primary fluorescence, accumulated vital dyes and lipid indicators in excess of other cells, and exhibited a more intense nadi reaction than other cells. Tests on fresh tissues are interpreted to indicate possible qualitative and quantitative differences in alkaloid content between subterranean and aerial portions of the plant and between mature and immature tissues. These studies showed that reactive products are unevenly distributed in cells and organs of the plant and can be microscopically detected only in laticifers and specialized parenchyma cells.

Reactions of Alkaloid and Histochemical Indicators in Laticifers and Specialized Parenchyma Cells of Catharanthus roseus (Apocynaceae)

Reactions of Alkaloid and Histochemical Indicators in Laticifers and Specialized Parenchyma Cells of Catharanthus roseus (Apocynaceae)

ONTOGENY AND STRUCTURE OF THE SECONDARY PHLOEM IN EPHEDRA

The secondary phloem in Ephedra is atypical of the gymnosperms in general and exhibits several angiosperm‐like characteristics. The ray system of the conducting phloem consists of parenchymatous, multiseriate rays. The axial system contains parenchyma cells, sieve cells, and unusual albuminous cells reminiscent of the specialized parenchyma cells found in some angiosperms. These cell types may intergrade with each other. P‐protein in the developing sieve element appears early in the form of a single, ovoid slime body. Later, smaller slime bodies appear and quickly disperse. In the mature sieve element the single, ovoid slime body is lost, and P‐protein is then evident in the form of a parietal cylinder, thread‐like strands, amorphose globules, or a slime plug. Necrotic‐appearing nuclei are commonly found in mature sieve cells.

STRUCTURAL EVIDENCE FOR A THEORY OF VEIN LOADING OF TRANSLOCATE

The ultrastructure of minor veins of Beta vulgaris was examined with reference to possible models for vein loading of translocate. Structural evidence was reviewed in the light of recent physiological observations as a basis for proposed mechanisms. Features which appeared to be of significance in formulating a model included the open, differentiated sieve plates, the predominance of organelle‐rich parenchyma cells, and the branched plasmodesmata connecting sieve tubes and parenchyma cells. The resulting model views cell to cell movement of photosynthate via the symplast to the specialized parenchyma cells. The actively accumulated sucrose appears to move from the specialized parenchyma cells into the sieve tubes via plasmodesmata in the lateral and end walls.

Vascularization of Fern Leaves

The subject of fern leaf venation is a very old topic and it has been dealt with by many authors during the nineteenth and twentieth centuries. Most of the discussions of fern leaf venation concentrate on its qualitative aspects resulting in descriptions of various types of leaf venation. Considerably less attention has been devoted to the quantitative aspects of fern leaf vascularization. This report will give a brief summary of some of the measurable aspects of the vascular system within the leaves of certain rather common ferns. The collections of the ferns used in this study came from two sources. Those collected out-of-doors came from Giant City State Park or from Jackson Hollow, six miles northeast of Simpson, Illinois. Material was taken with the permission and under the supervision of Dr. Robert H. Mohlenbrock of Southern Illinois University. Other ferns were obtained from the Department of Botany greenhouse at the University of Nebraska. The fern leaves were studied from slides made of cross sections of the material which had been stained with Delafield's hematoxylin and safranin. Whole mounts of cleared leaves or leaflets or parts of these were also studied to obtain vein patterns and angle of branching. All measurements recorded in this study were made by means of an ocular micrometer. All those listed in Table I are averages of many separate measurements. Leaf thickness was measured at veins rather than between veins. Intervascular interval was measured from the vascular tissue of one bundle to the vascular tissue of the next adjacent bundle; thus it includes any specialized parenchyma or bundle sheath cells. Vein width includes only the width of the conductive tissues of a vascular bundle. Angle of branching refers to the branching of vascular bundles from the midvein of the leaf or segment and not to the ultimate branchings in the lamina. It was measured in linear units and the angle 87

Ultrastructure of Specialized Parenchyma Cells in the Leaf Blades of the Sensitive Plant Mimosa Pudica L

The ultrastructure of specialized parenchyma cells which are believed to conduct an electrical stimulus as a membrane action potential is described for M. pudica.

The Anatomy of the Infra-red Sense Organ in the Facial Pit of Pit Vipers

ABSTRACT The histological composition, of the sensory membrane in the facial pit of rattlesnakes (Crotalus spp.), the disposition of the nerve-trunks entering it, the fibre-size spectrum of the nerves, and the form of the sensory endings are described. Between the two layers of extremely attenuated epidermis the principal constituent of the membrane is a single layer of specialized parenchyma cells with osmiophil, reticular cytoplasm. These are not regarded as sense cells but they react strongly and locally to degeneration of nerve-endings. The axons enter through numerous trunks from three branches of the trigeminal nerve, from all sides of the membrane, providing a dense innervation. They lose their myelin, taper to about 1 p, then expand into flattened palmate structures which bear many branched processes terminating freely over an average area of about 1,500 p.2, overlapping only slightly with adjacent units but leaving virtually no area unsupplied. This means there are from 5 00 to 1,5 00 axons ending per mm2, an estimate which agrees with the nerve-counts. No other form of ending was found. The mode of the fibre-size spectrum lies in the region 5-7p diameter. A transmission spectrum of the fresh membrane shows broad absorption peaks at 3 and 6 p and about 50% transmitted in other regions out to 16p. The visible spectrum is at least 50% transmitted and probably much is lost by reflection. Strong absorption takes place at wavelengths shorter than 490 p. The anatomical adaptations of the sense organ are discussed, especially the concentration of warm receptor fibres, the thinness of the membrane, the extremely superficial position of the nerve-endings—all increasing sensitivity to caloric flux. The overhanging margins of the pit and the richness of supply are believed to permit directionality of reception. It is suggested that the palmate form of the ending has a significance in permitting several independent local subthreshold activity generators to coexist in the processes and in pooling their coincident, electrotonically spread potentials to influence the initiation of spikes which may take place at the junction of axon and palm.