The discovery of arbuscules in Aglaophyton major, an Early Devonian land plant, provides unequivocal evidence that mycorrhlizae were estabised >400 million years (Fig. 1), even though the intraradical mycelium is extensively developed within the intercellular spaces in the remaining cortical tissues. The nonseptate hyphae range up to 4.0 ,um in diameter and often show both Y-and H-shaped anastomoses. In extant VAM fungi arbuscules are formed when intercellular hyphal branches penetrate the cell wall of the host but do not rupture the plasmalemma (10). The hyphal trunk ofthe arbuscule is =1.3 Am wide (Fig. 2) and branches repeatedly to form a "bush-like" structure within the cell (Fig. 3). Secondary branches range from 0.7-2.0 pum wide, and the ultimate ones are beyond the resolving power of light microscopy (in the size range 0.2-0.5 pum). Fig. 4 shows the top of an arbuscule indicating the configuration ofthe more distal branches. In the arbuscules of extant VAMs, the walls are osmiophilic and decrease in thickness to <20 nm near the tips. In some arbuscules the tips may be slightly swollen, and this condition also occurs in the fossils.In extant VAMs, the arbuscules are ephemeral and begin to break down at the tips ofthe smallest branches in 4-6 days, ultimately forming an amorphous mass within the cell (11). We have no way of gauging the longevity of the fossil arbuscules except to note that various stages of arbuscule morphology are present in a single section, including those that have collapsed and deteriorated. Boullard (12) suggested that in some living ferns the frequent presence of clumps indicates a short functional span of an arbuscule. Several authors working with modern VAM fungi reported the formation of a subapical septum that separates the functional and nonfunctional portion of an arbuscule during deterioration. We have not observed this structure in any of the fossil arbuscules to date. The fossil arbuscules appear morphologically identical to those of many extant VAMs (13). There is far less information available about the structural and physAbbreviation: VAM, vesicular arbuscular mycorrhiza.fTo whom reprint requests should be addressed. 11841The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
The leaves of extant terrestrial plants show highly diverse and elaborate patterns of leaf venation. One fundamental feature of many leaf venation patterns, especially in the case of angiosperm leaves, is the presence of anastomoses. Anastomosing veins distinguish a network topologically from a simple dendritic (tree-like) pattern which represents the primitive venation architecture. The high degree of interspeci®c variation of entire venation patterns as well as phenotypic plasticity of some venation properties, such as venation density, indicate the high selective pressure acting on this branching system. Few investigations deal with functional properties of the leaf venation system. The interrelationships between topological or geometric properties of the various leaf venation patterns and functional aspects are far from being well understood. In this review we summarize current knowledge of interrelationships between the form and function of leaf venation and the evolution of leaf venation patterns. Since the functional aspects of architectural features of dierent leaf venation patterns are considered, the review also refers to the topic of individual and intraspeci®c variation. One basic function of leaf venation is represented by its contribution to the mechanical behaviour of a leaf. Venation geometry and density in¯uences mechanical stability and may aect, for example, susceptibility to herbivory. Transport of water and carbohydrates is the other basic function of this system and the transport properties are also in¯uenced by the venation architecture. These various functional aspects can be interpreted in an ecophysiological context.
Summary• The Early Devonian Rhynie chert has been critical in documenting early land plant-fungal interactions. However, complex associations involving several fungi that enter into qualitatively different relationships with a single host plant and even interact with one another have not yet been detailed.• Here, we studied petrographic thin sections of the Rhynie chert plant Nothia aphylla.• Three fungal endophytes (co)occur in prostrate axes of this plant: narrow hyphae producing clusters of small spores; large spherical spores/zoosporangia; and wide aseptate hyphae that form intercellular vesicles in the cortex. Host responses on attack include bulging of infected rhizoids, formation of encasement layers around intracellular hyphae, and separation of infected from uninfected tissues by secondarily thickened cell walls.• A complex simultaneous interaction of N. aphylla with three endophytic fungi was discovered. The host responses indicate that some of the mechanisms causing host responses in extant plants were in place 400 million yr ago. Anatomical and life history features of N. aphylla suggest that this plant may have been particularly susceptible to colonization by fungi.
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