A. W. Robards

The structure of barley roots in relation to the transport of ions into the stele

SummaryThree successive states are recognized in the development of endodermal cells in seminal and nodal axes and primary lateral roots of barley: 1. Casparian bands in the anticlinal walls; 2. suberin lamella around the whole inner face of the wall; 3. unevenly deposited cellulosic wall thickening. These states develop asynchronously, the cells adjacent to the protoxylem pole cells always being last to mature. All cells have progressed to at least the secondary state by 32 cm from the tip in seminal axes, 48 cm from the tip in nodal axes, but only 6 cm from the tip in primary laterals. The asynchronous development gives the appearance of “passage” cells adjacent to the protoxylem pole cells, although all cells eventually attain the same state and degree of wall thickening. Long distance transport of calcium shows a close correlation with the incidence of suberin lamellae in the three types of root examined; it is suggested that formation of a suberin lamella effectively blocks calcium movement into the stele and, therefore, long distance transport. Plasmodesmata are present in comparable frequencies through both tangential and radial endodermal walls; they appear to maintain intercellular continuity until a late stage in development.

A correlated histochemical and ultrastructural study of the epidermis and hypodermis of onion roots

SummaryCell walls of mature epidermal and hypodermal cells are autofluorescent when viewed under ultraviolet or blue light. This autofluorescence develops in a centripetal direction, beginning in the outer tangential wall of the epidermis and ending in the inner tangential wall of the hypodermis. The intercellular regions between the epidermis and hypodermis and between the hypodermis and the cortex are dense and also become autofluorescent. Although the walls of the hypodermis provide a barrier to the movement of a high molecular weight fluorescent dye, the walls of the epidermis are permeable. Histochemical studies indicate that lipids and polyphenolics are components of the epidermal and hypodermal cell walls. Both layers are resistant to the wall-degrading enzyme Driselase and to concentrated sulphuric acid, whereas the cortex is digested with both treatments. Observations with the transmission electron microscope show that a complex suberin lamella encases each hypodermal cell but is absent from the epidermis. However, the outer tangential wall and radial walls of the epidermal cells are complex in that layers of different densities are present. Some of these layers, as well as the intercellular regions and the radial walls of the hypodermal cells, bind ferric ions when tissue is fixed in ferric chloride-glutaraldehyde indicating the presence of poly-phenolics in these regions. An extracellular layer covering the outer tangential wall of the epidermis stained positively with a number of histochemical tests for polyphenolics.

The ultrastructure of plasmodesmata

SummaryIt is suggested that the central strand which traverses plasmodesmata is in open continuity with the endoplasmic reticulum of adjacent cells, and that this strand (desmotubule) represents a modulation of a normal ER membrane so that it comprises solely spherical protein subunits. This concept is used to illustrate how plasmodesmata could form a median nodule or anastomosing central strands. The implications of this model in relation to current theories of symplasmic transport are discussed, and the possibility for further experimental work is outlined.

On the ultrastructure of differentiating secondary xylem in willow

SummaryStudies of differentiating xylem inSalix fragilis L. show the immediate cambial derivatives to be ultrastructurally similar. The Golgi apparatus is important at all stages of wall synthesis, possibly producing (amongst other substances) hemicellulose material which is carried to the wall in vesicles or multivesicular bodies. The endoplasmic reticulum also contributes one or more components to the developing wall: at some stages during differentiation the endoplasmic reticulum produces electron opaque bodies which appear to be guided towards the wall by microtubules. Compact structures formed from concentric membranes (myelin-like bodies) have been found joined to rough endoplasmic reticulum, but their presence is not explained.Two types of plasmalemma elaboration occur: invagination of the plasmalemma itself to form vesicles which may contain cytoplasmic material; and vesicles between the plasmalemma and cell wall which are the result of single vesicles or multivesicular bodies traversing the plasmalemma. Both systems provide a means for transporting cytoplasmic material across the plasmalemma.Microtubules have been seen associated with all vesicles derived from the cytoplasm which appear to be moving towards the wall. The presence of microtubules may generally be explained in terms of orientation of vesicles, even if they also happen coincidentally to parallel the supposed orientation of microfibrils in the wall itself. It is possible to resolve connections between the microtubules and the plasmalemma.

Structure and permeability of the epidermal/hypodermal layers of the sand sedge (Carex arenaria, L.)

SummaryDuring the normal course of development in the root of sand sedge (Carex arenaria), many of the cells in the central cortex degenerate to form an extensive aerenchyma. The epidermis and multi-layered hypodermis become heavily suberized and only remain attached to the stele by a few, thin, radial files of cortical cells that have not autolyzed. These changes make it possible for epidermal/hypodermal sleeves to be isolated so that their permeability can be determined. The coefficient of diffusive permeability for tritiated water is of the order of 20 nm s−1, while the permeability coefficient for self diffusion of labelled phosphate and calcium is approximately 5.0 and 4.0 nm s−1 respectively. These values are approximately 200 (water) and 500 (phosphate and calcium) times lower than similar determinations made on isolated sleeves of onion roots. The outer layers of the mature root ofC. arenaria are thus extremely impermeable: any significant absorptive function has been lost at the expense of a protective role provided by the modified cortex for the vascular stele.

Nectar secretion inAbutilon: a new model

SummaryNectary trichomes ofAbutilon striatum secrete copious amounts of sucrose, fructose and glucose. The nectar emerges from transient pores in the cuticle overlying the trichome tip cells. Calculations of the required transmembrane fluxes, either across the tip cell plasmalemma or across the cell membrane of the whole trichome, give very high rates compared with those obtained from other situations in plants and, therefore, cast doubt on the possibility that nectar secretion inAbutilon is an eccrine process. Quantitative evaluation of the possibility of granulocrine secretion, by successive fusion of vesicles with the cell membrane, suggests that this is an even less probable mechanism of secretion. Rapid freezing followed by freeze-substitution or freeze-fracture replication reveals that an extensive “secretory reticulum” (SR) is present within the hair cells. As similar micrographs are obtained from conventional, chemical fixation it is argued that the secretory reticulum is a relatively stable endomembrane system. Freeze-fracture and freeze-substitution micrographs show that this internal membrane system is closely associated with the plasmalemma. Taken together with other structural information, as well as physiological data, it is concluded that prenectar is actively loaded into the secretory reticulum of all trichome cells. Increase in hydrostatic pressure within this compartment leads to the opening of “sphincters” which connect the cisternal space of the SR to the outside of the plasmalemma. Thus a pulse of nectar is forcibly expelled into an apoplastic compartment sealed to the outside by the impermeable cuticle and on the inside by the plasmalemma. As this apoplastic compartment is also sealed at the stalk cell, the only route for pressure release is via the transient pores which overlay the tip cell. Distension renders these patent so that, again, pulsed secretion is observed. This hypothesis overcomes the necessity for envisaging excessively high transmembrane fluxes or rates of vesicle fusion. It would imply the need for a continuing supply of prenectar to the hair cells accompanied by active loading into the SR. This loading process may well be supported by the hydrolysis of sucrose to glucose and fructose and is probably the site where ions and other low molecular weight solutes are filtered from the nectar.

Rose leaf structure in relation to different stages of micropropagation

SummaryThe Mme Isaac Pereire rose was investigated in an attempt to establish how micropropagated roses might best be weaned into normal growth conditions. Leaves of in vitro grown plants, weaned plants and the stock plant were studied, using light microscopy and different scanning and transmission electron microscopical techniques. Features that varied in the different growing conditions were leaf size and thickness, amount of wax, thickness of cuticle and external epidermal cell wall, number and aperture of the stomata, size of the epidermal cells, number of layers of the palisade cells, and size of the chloroplasts in the mesophyll. The rose in the present study had wax on the in vitro cultured plants; this wax was of similar ultrastructural appearance to that of the stock plant, even though in smaller quantities. Weaned plants had an intermediate amount of wax. The cuticle was thin, ranging from 0.04 μm on plants growing in vitro to 0.3-0.6 μm on weaned plants and stock plants. Stomata were always wide-open on leaves taken from cultures with a relative humidity of 100%. After four weeks in a humidity lowered to 85% stomata had closed.

Sugar secretion from the nectary ofStrelitzia: an ultrastructural and physiological study

SummaryFlowers ofStrelitzia reginae grown at a constant 20°C have been shown to secrete nectar at a rate of up to 5.0 mg (d.w.) sugar h−1 (mean rate 1.2±0.1 mg h−1) for up to seven days. The nectar has a total concentration of about 25% during the early part of the secretory period but often falling to less than 10% towards the end of secretion.Each flower has three septal nectaries, the cuticle-lined ducts of which open into a nectar basin formed by the fused bases of two petals on the top of the receptacle. The layer of epithelial cells which secretes the sugars is thrown into highly convoluted folds and the distal parts of these cells have profuse wall inpushings. Both of these modifications have the effect of increasing the surface area of the plasmalemma apparently available for unloading the nectar. The glandular epithelium of the three, 26 mm long, nectaries of a single flower would be lined by more than 17×106 cells with a total plasmalemma surface area for unloading of at least 2,000 mm2. There is little evidence to suggest that secretion is a granulocrine process inStrelitzia. While there is abundant, stacked endoplasmic reticulum, and numerous vesicles containing fibrillar material, these do not appear to be directly concerned with sugar secretion. Data from specific flowers suggest that transmembrane fluxes in the range of 1.0×106 to 1.0×10−7 mol s−1 m−2 would be necessary to sustain the observed rates of secretion. While these are relatively high, when taken together with the structural information, they lead to the conclusion that secretion inStrelitzia is probably an eccrine process.

Effects of chilling temperatures on root cell membranes as viewed by freeze-fracture electron microscopy

SummaryThe cortical cell membranes of maize and marrow roots grown at normal, or chilling, temperatures have been studied by freeze-fracture electron microscopy. Using computer-assisted methods to analyse intramembraneous particle (IMP) frequencies, diameters and distribution, no significant trends in differences between normal and chilled roots were found. While this result does not correspond with the findings from similar experiments on microorganisms, it is compatible with contemporary ideas concerning temperature-induced phase transitions in the lipids of higher plant cell membranes. The cortical cell membranes of barley roots that had been subjected to cold osmotic shock also showed no differences from untreated roots as demonstrable in freeze-fracture replicas.IMPs were found to cluster around plasmodesmata after chilling but the physiological significance of this, if any, remains to be investigated further.While these negative results only indirectly help towards understanding how cell membranes react to chilling, the techniques described open the way for more detailed analyses of IMP characteristics in plant cell membranes.

Observations of the pattern of secondary wall development in the hypodermis of onion (Allium cepa) roots

SummaryThis brief communication reports the appearance, under certain circumstances, of root hypodermal cells with transfer cell labyrinths. These cells lie at regular intervals around the hypodermis at the bases of onion bulb roots. They are narrower (smaller tangential dimension) than unmodified cells but have the same radial dimension. These “narrow cells” contain small vacuoles, their main volume being composed of a cytoplasm rich in organelles, especially mitochondria. When treated with a low concentration of lanthanum nitrate solution, the tracer accumulates in the outer tangential wall and in small vacuoles and vesicles.