In this article we will discuss about the physical association between two plants for mutual benefit.
(a) Lichen:
The well-known association between algae and fungi is the lichen. The lichens are of various forms and grow in various habitats like rocks, trees etc., but never grow in highly polluted areas like heart of most cities, etc.
Though they live in mutual association, according to some workers, the’ fungal partners dominate over the algae and algae live as subordinate partner. Such association is called helotism. This type of association has great economic value in respect of food, fodder and medicine, ecological balance, etc., e.g., Graphis, Haematomma, Parmelia, Peltigera, Usnea, Cladonia, etc.
(b) Root nodules:
It is the well-known association between the roots of leguminous plants and nitrogen-fixing bacterium, Rhizobium (Fig. 1.19). After the death of the leguminous plants, the stored nitrogen in the nodule increases the soil fertility after its decay.
(c) Mycorrhiza:
It is the association between fungi and the root of higher plants. Various types of mycorrhizal association are now recognised. Two common types are Ectomycorrhiza (when the fungus lives mostly outside the root tissue) and Endomycorrhiza (when the fungus grows mostly inside the root cortex).
The fungus helps the plant to get more nutrients in exchange of small amount of food required for its living. The fungus never takes the entry inside the protoplasm, but remains in- between cell wall and cell membrane.