First of all the mycorrhizae are fungi, not a portion. Pro'lly you mean somethin more like, what is the function of the fungal portion of a mycorrhizal symbiotic relationship ? ( mycorrhiza is the singular, mycorrhizae is the plural, and singular or multiple it is hard for me to keep spelling) The symbiotic relationship is between a fungus and a "green" plant. Fungi do not produce their own 'food' thru photosynthesis like green plants, so in this symbiotic relationship, the fungus gets food from the green plant. In return the fungus provides the green plant with some minerals which the green plant cannot otherwise access, some protection against some diseases, and water from dryer soils. One reason the fungus can provide these benefits for a green plant is because of the differences in the chemistry of the two plants. Many mineral nutrients in soil can become bonded to other chemicals, iron is especially active and can 'trap' other chemicals, fungi can break that bond easier than green plants. In addition the fungus produces other chemicals that are either resistant or even toxic to other organisms, bacteria or even other fungi, that could attack green plants. Perhaps more important is the physical anatomy of the fungus. Fungi do not have 'roots', the actual 'body' of the fungus is called the mycelium and it is made up of, among other structures, hyphae , or very small tube like structures. The hyphae are much narrower than even the finest root hairs of green plants and can get into places that real roots can't, even into microscopic cracks and openings in 'solid' rock, also a given number of hyphae, which would equal in size a root hair, have a larger surface area than that one root and so a larger absorption area for nutrients and water. The fungus can also just be able to reach distances the green plant can't. The ratio of hypahe to the entire fungus is much greater than the ratio of roots to the entire green plant, for example a 'mushroom' that pops up in your yard is the 'fruiting body,' or spore producing part, of mycelium that can be spread out over several dozen feet. In fact a mushroom in one corner of your yard may be from the same mycelium as another mushroom clear across the yard. Some mycelium can have an area of acres. There are two main types of mycorrhizae; Endomycorrhizae ('inside'mycorhizae), where the hyphae actually grow into the cells of the plants root, and Ectomycorrhizae ('outside'mycorhizae) where the hyphae grow between the roots, and nutrients are exchanged thru the cell walls as if they were just being absorbed directly from the soil. I would give you some examples of mycorrhizal relationships but it would be pointless, since at least 95% of all plant families are predominantly mycorrhizal. The relationships are about 400 million years old, going back to when plants began to move out of the water onto dry land, the fungi were already there trying to survive and just waiting for a source of food. The plants were glad to meet them cause it was now harder to collect nutrients and water. Next time please ask about sumpin easier to spell like peas, or the relationship between nuts and squer, squieer, squerul...tree rats. BTW I recently attended a family reunion/cruise on a relation ship. My cousins played some great music on their hyphae, they were some real funguys. But Aunt Mycellia just sat around like a toadstool, but that might have been because there was a corn on her toe trying to form an association with the fungus of her athletes foot.

Actually, its easier put that, the mycorrhizae is the root system for the fungus that provides a large surface area for absorbing nutrients.