Mycorrhiza orchid mycorrhizae

In his discussion of endotrophic mycorrhizae, Hacskaylo states that normally unless a specific fungus penetrates the orchid embryo, seed germination does not occur. It is of interest to know that some of these same fungi are serious pathogens on other hosts. Fig.S.2, taken from Hacskaylo s article, shows the appearance of endotrophic mycorrhizae of red maple. In this case the invaded roots are beaded as a result of periods of arrested growth followed by periods of active growth. 

Plants and fungi often co-operates. The most well known example of such cooperation is the lichens where an algae and a fungus cooperates and build up the lichen body. Most land plants are also dependent on co-operation with plants. The cooperation takes place between the plant roots and the fungus growing out into the soil. In most cases the symbiosis is mutualistic in that both partners gain on the relationship but it is not necessary (as with the orchid mycorrhiza. 

There are several types of mycorrhiza. The three most important types are the arbuscular mycorrhizas (AM) of herbaceous plants and some tropical trees, the ectotrophic mycorrhiza of forest trees and the endotrophic mycorrhiza of orchids. This division is based on the morphological characteristics of the mycorrhiza (fungal-filled root) (Fig 6.1). 

Rhizoctonia solani have been shown to form mycorrhiza with orchids under experimental conditions. Since the relationship between the plant and the fungus is not mutualistic this type of mycorrhiza is not considered further. 

Gehlert, R. and H. Kindl, Induced formation of dihydrophenanthrenes and bibenzyl synthase upon destruction of orchid mycorrhiza. Phytochemistry, 30, 457-460 (1991). 

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