IV. VESICULAR ARBUSCULAR MYCORRHIZAE
|Cross sections of Aglaophyton axes often show a peculiar dark band, located at three to six cells below the epidermis. The darker colour is not caused because cells have thicker walls, but at higher magnifications it becomes clear that it is related to the presence of mycorrhiza. The intracellular cavities are often completely filled with fungal hyphae. The fungi lived in symbiosis with the plant. Such symbioses are very common in extant plants. Both the plant and the fungus benefit from this mutual relationship. The fungus extracts organic nutrients from the plant, whereas the latter gets water and minerals via the fungus. Mycorrhiza fungi living inside the cells of the plants are called endomycorrhiza. After having penetrated the cell wall and having entered the cell the fungus normally forms tree-shaped outgrowths, the so-called arbuscles, inside the cells of the host plant. The exchange takes place at these arbuscles. Although probably 90% of all modern plants have mycorrhiza, these are very difficult to demonstrate in fossil material due to their delicate nature. Arbuscles are only short-living (up to three days) before they desintegrate as a result of the reaction of the host plant. There are onyl very few examples of vesicular arbuscular mycorrhizae (= VAM) in the fossil record, but those associated with Aglaophyton are unequivocal. The fact that several growth stages and the typical host reaction can be demonstrated, proves that the cells in which arbuscles are found were living cells, i.e., that these fungi were neither saprophytic nor parasitic but really symbiontic. The fact that all growth stages of these very short-lived arbuscles are found indicates that the silicification process must have taken place almost instantaneously. The entire cycle from the penetration of the cell, to formation of an arbuscle and finally the decay takes only few days, usually two or three. VAM have originally been described for Aglaophyton, but also some other Rhynie Chert plants have these symbiontic fungi. Also on the overview of the Rhynia axis illustrated on this website they are clearly visible. The mycorrhizae associated with Aglaophyton have been described as Glomites (Glomaceae). Although it has repeatedly been suggested that mycorrhiza played an important role in the colonization of the land, the Rhynie Chert has first shown that such symbioses were well established as early as the Early Devonian.|
|Above: Fungal hyphae penetrating a lying axis of Aglaophyton;
the cuticle of the axis is to be seen below.
Middle: Detail of the cortical zone with fungal hyphae (darker) in the intercellualar spaces.
Below: A few cells with well developed arbuscles; even the places where the fungus penetrated the cell wall are clearly visible.
|© Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster||