During gel (gum) formation in angiosperm trees, fibrillar material accumulated in protective layers of xylem parenchyma cells before being secreted across half-bordered pit membranes into vessel elements. Immunogold labeling demonstrated that this fibrillar material was mainly composed of partially esterified pectic polysaccharides. The primary wall of expanding tyloses, an extension of the parenchyma protective layer, secreted similar pectic substances to completely block vessel elements. In most studies, these occluding structures were reported to be formed in response to causative factors such as aging processes, injuries, or infections. Current observations support the view that partial to complete embolism, which almost always accompanies these factors, might be the main cause triggering the formation of vessel occlusions. Whereas pectin seems to be the basic component of gels (gums) and of the external layer of tyloses, other substances, such as phenols, were also detected either as a part of these plugs or as accumulations beside them in vessels. Finally, it is proposed that the term 'gel' instead of 'gum' be used in future studies to describe the occluding material secreted by ray and paratracheal parenchyma cells.

The colonization processes of the xylem in the susceptible carnation cv. Early Sam and the resistant cv. Novada were studied ultrastructurally following inoculation with Fusarium oxysporum f.sp. dianthi. Samples from 1 to 3 cm above the incision were collected over 5 weeks and processed following conventional procedures as well as with probes for cellulose, N-acetyl-glucosamine, and pectin. The fungus grew profusely in the vessel lumina of the susceptible cultivar. Some of the colonized vessels were lined with coating material connected to the fungal cell wall and extending into the host cell wall through microfilamentous-like structures. Coatings did not label for pectin or cellulose. The pathogen crossed from one vessel element to another (and at times to parenchyma cells) usually directly through pit membranes; often the invading structures of the fungus appeared to be either only membrane-bound or formed solely of microfilamentous-like entities. The fungus subsequently invaded extensively, generally by means of microhyphae, the vessel intercalary walls from the pit membranes and vessel wall junctures. Microhyphae had thin or imperceptible walls and contained only some of the normal cytoplasmic components. Initially, the invading hyphae dislocated the host cell walls, apparently mechanically more than by lysis; however, more pronounced lysis occurred following general tissue invasion. Host parenchyma cells seemed relatively unaffected, even after the surrounding walls had undergone severe degradation. Colonization of resistant plants was restricted. Degradation of tissues did not occur and microhyphae were not observed. Inoculated vessel elements in the 'Novada' plants contained numerous fungal cells and little occluding material, whereas the surrounding vessels were almost completely occluded. The initially invaded xylem became tangentially compartmentalized by parenchyma cell wall thickenings and by hyperplastic parenchyma. Occasionally, hyperplastic tissues were slightly re-invaded, forming secondary invasion pockets. Vessel-occluding material varied in structure and opacity, not only from vessel to vessel but also within the same vessel, and contained microfilamentous-like structures and other types of fine fibrillar material. Some vessel elements in or near the secondary invasion pockets contained only the finer fibrils that reacted strongly with an antibody specific for pectin. Vessel elements rarely contained tyloses.Key words: cellulose, chitin, Dianthus caryophyllus, Fusarium wilt, gels and gums, host wall degradation, microhyphae, pectin, tyloses.

R~o u x , D., and OUELLEWE, G. B. 1991. Barrier zone formation in host and nonhost trees inoculated with Ophiostoma ulmi. I. Anatomy and histochemistry. Can. J. Bot. 69: 205552073, Barrier zone formation was studied in small branches of Ulmus americana L., Prunus pensylvanica L.f., and Popillus balsamifera L. following inoculation with Ophiostoma ulmi (Buism.) Nannf. (the Dutch elm disease pathogen). Barrier zones were continuous in the nonhosts whereas they were generally discontinuous in U. americana; barrier zone formation also occurred at a later stage of infection in the latter than in the former. Barrier zones were formed of parenchyma cells and fibers in U. americana, mainly of parenchyma cells in Prunus pensylvanica, and of fibers in Populus balsamifera. Fibers as a principal component of barrier zones are described for the first time. Histochemical tests revealed that the proportion of lignin was higher in barrier zone cell walls than in elements of the noninvaded xylem. Barrier zones contained suberized cells, the number of which was progressively greater in the order U. americana, Prunus perzsylvanica, and Populus balsamifera. However, many fibers of U. americana occasionally formed a continuous barrier zone and had an internal layer that was slightly suberized. In addition, phenolic compounds were usually detected within barrier zone cells of these species.

A comparative study of isolates and their corresponding herbarium specimens of Gremmeniella spp. from pine, spruce, and balsam fir, of Ascocalyx laricina from larch, and of Ascocalyx abietis from balsam fir was conducted using morphological and cultural characteristics as well as electrophoresis tests on 12 and 10 – 15% gradient polyacrylamide gels. In addition, the conidia and the ascospores of the specimens were treated with 4′,6-diamino-2-phenylindole to determine the number of nuclei per cell. Ascocalyx abietis is quite distinct in the morphology of its anamorph and in its cultural characteristics from all other samples studied. The specimens from larch differ from the others by their 2-celled ascospores and their cultural characters. Among specimens from pines and from Picea and Abies, minor morphological and cultural differences were found. The electrophoresis tests confirm the differences observed in the cultural studies. The statistical analysis of the results supports the following classification of the specimens studied: within Gremmeniella abietina, two varieties are recognized, a var. typical, with the known "European" and "North American" races from Pinus and the "Asian" race from Abies sachalinensis, and a new variety, var. balsamea, comprising the isolates from Picea and Abies (Québec). A new combination is proposed in Gremmeniella for Ascocalyx laricina. Ascocalyx abietis is retained as a separate genus.

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