Richard P. C. Johnson

Epicuticular wax in the stomatal antechamber of sitka spruce and its effects on the diffusion of water vapour and carbon dioxide

SummaryThe distribution of wax tubes on the leaf surfaces is described, especially the presence of wax tubes in the antechambers of the stomata. The extra resistances which the wax-filled antechambers add to the other resistances in the pathway for diffusion of water vapour and of carbon dioxide are calculated. We conclude that the wax-filled stomatal antechambers reduce the rate of transpiration by about two thirds but reduce the rate of photosynthesis by only about one third. Thus wax-filled stomatal antechambers are excellent antitranspirants.

Microfilaments in pores between frozen-etched sieve elements.

SummarySieve tubes were frozen before being cut from plants and were prepared for electron microscopy by freeze-etching. Structures that may be interpreted as filaments appeared in and near pores through sieve plates. Their presence suggests that filaments seen in sieve-pores prepared chemically may be there normally. Filaments appeared more numerous and compacted in sieve pores between sieve elements that had been pre-treated with glycerol than in those that had merely been frozen. A sieve element treated with glycerol appeared plasmolysed. No evidence was found for membrane-bound transcellular strands through pores in sieve plates even though membrane-bound transvacuolar strands of cytoplasm appeared clearly in nearby parenchyma cells.

Immunocytochemical localisation of phloem lectin from Cucurbita maxima using peroxidase and colloidal-gold labels

Antibodies were raised against lectin purified from the sieve-tube exudate of Cucurbita maxima. Immunocytochemistry, using peroxidase-labelled antibodies and Protein A-colloidal gold, was employed to determine the location of the lectin within the tissues and cells of C. maxima and other cucurbit species. The anti-lectin antibodies bound to P-protein aggregates in sieve elements and companion cells, predominantly in the extrafascicular phloem of C. maxima. This may reflect the low rate of translocation in these cells. Under the electron microscope, the lectin was shown to be a component of P-protein filaments and was also found in association with the sieve-tube reticulum which lines the plasmalemma. The anti-lectin antibodies reacted with sieve-tube proteins from other species of the genus Cucurbita but showed only limited reaction with other genera. We suggest that the lectin serves to anchor P-protein filaments and associated proteins to the parietal layer of sieve elements.

Fine Structure of the Host-Fungus Interface in Orchid Mycorrhiza

SummaryElectron microscopy of protocorms of Dactylorhiza purpurella infected with a symbiotic Rhizoctonia sp. showed that the intracellular hyphae examined did not penetrate the plasmalemma of the host cell. Walls of hyphae within cells bore many hemispherical protuberances over which the host plasmalemma was closely pressed. we estimate that these protuberances would increase the area of contact between hyphae and host plasmalemma by about 15%. They were not found on hyphae growing on agar. Except for these protuberances, and some vesicles or tubules which invaginated the fungus plasmalemma, no other structures were seen which could be suggested to be adaptations to transport across the living fungus-host interface.

Negative stain in wax tubes from the surface of Sitka spruce leaves.

SummaryCarbon replicas and scanning electron micrographs of wax outgrowths on some leaf surfaces have not shown conclusively whether the outgrowths are tubes or solid rods. Some workers have suggested that they are solid rods. We find that negative stains will penetrate into a cavity in the centre of wax outgrowths on leaves of Sitka spruce (Picea sitchensis Bong. Carr) and Tulipa kaufmanniana Elliot. Thus these outgrowths appear to be tubes.

The microscopy of P-protein filaments in freeze-etched sieve pores

Intact vascular bundles from Nymphoides peltata (S.G. Gmel.) O. Kuntze, shown to have translocated carbon-14, were freeze-fractured and etched for electron microscopy. The interpretation of freezefractured and etched sieve pores and P-protein filaments seen in them is discussed. The entire widths of most of the sieve pores seen contained filaments separated by less than 100 nm. Their arrangement indicates too high a resistance to flow for pressure flow alone to drive translocation at known rates; pumps would be necessary at places along sieve tubes. However, calculations are presented to show that during the time taken to fix pores, by fast freezing or chemically, the filaments in them could rearrange and move further by Brownian and other motion than the distances between filaments which we need to measure. These calculations show that it is not possible, by microscopy alone, to answer the outstanding question “How are filaments arranged in translocating sieve pores?” with enough certainty to tell us whether pressure flow is adequate to explain translocation where filaments are present. The calculations are relevant also to microscopy of other cell structures which may move.

Crystalline fibrils and complexes of membranes in the parietal layer in sieve elements

SummaryFibrils are described which were found in the parietal layer in well differentiated sieve elements under the electron microscope. The fibrils were usually about 0.1 micrometers wide and appeared, in longitudinal section, to be composed of closely parallel filaments. In transverse section they appeared as a lattice, possibly of closely packed tubules. Some of the fibrils were joined to complexes of membranes in the parietal layer and to plasmatic filaments. Their relationship to these structures and to fibrils which may cause movements in other kinds of cells is discussed.

Endoplasmic reticulum and crystalline fibrils in the root protophloem of Nymphoides peltata

Endoplasmic reticulum in the root protophloem of Nymphoides peltata (S.G. Gmel.) O. Kuntze changes form as sieve elements differentiate. In immature sieve elements the individual endoplasmic reticulum (ER) cisternae form large irregular aggregates in the cytoplasm. In older immature sieve elements the ER aggregates are more ordered and membranes in them are convoluted. Although convoluted ER predominates in immature sieve elements the ER of the mature sieve elements consists mainly of flattened stacks of ER cisternae. Some of these stacks of ER may be derived from the existing convoluted ER. “Crystalline fibrils” first appear in the cytoplasm of the sieve element when the ER starts to aggregate. The crystalline fibrils move to the parietal layer of the sieve element along with the aggregates of ER. A possible ontogenetic relationship between ER and crystalline fibrils is discussed.

Can cell walls bending round xylem vessels control water flow

Vascular bundles of petioles below wilted leaves of Nymphoides peltata (S.G. Gmel. O. Kuntze) were frozen intact and freeze-fractured for electron microscopy. Cell walls in them appeared drawn in against the helical thickenings of xylem vessels. By contrast, walls round vessels which had been frozen in vascular bundles below turgid leaves, and walls round vessels which had been fixed, embedded and sectioned, were straight or bulged outwards slightly. Walls bulged outwards slightly also from cut vessels filled with sucrose solution before freezing. Movement of vessel walls could produce the clicks audible when water cavitates in vessels, and might explain a variable resistance to the flow of water through plants.

POTASSIUM PERMANGANATE FIXATIVE AND THE ELECTRON MICROSCOPY OF SIEVE TUBE CONTENTS

SummarySieve tube “slime” is probably fibrillar as suggested by other workers. The granular material seen in electron micrographs of sieve tubes “fixed” with potassium permanganate is mostly a precipitate produced by the reaction of the permanganate with the sieve tube contents, particularly with sucrose, and with reagents used in the fixing, dehydrating and embedding process. Potassium permanganate is not therefore a good fixative for the electron microscopy of sieve tube contents. Neither is it suitable for the study of fine structure in vacuoles or vesicles containing precipitable solutes in other kinds of cells nor for examining the fine structure of their ground cytoplasm.