Delphinidin

More than three-quarters of all seed plant species have various fungi associated with their roots. The association is mutualistic; that is, both the fungus and the root benefit from it and are dependent upon the association for normal development.
The fungus is able to absorb and concentrate phosphorus much better than it can be absorbed by the root hairs. In fact, if mycorrhizal fungi have been killed by fumigation or are otherwise absent, many plants appear to have considerable difficulty absorbing phosphorus, even when the element is abundant in the soil. The phosphorus is stored in granular form until it is used by the plant. The fungus also often forms a mantle of millions of threadlike strands that facilitate the absorption of water and nutrients. The plant furnishes sugars and amino acids without which the fungus cannot survive.

These “fungus-roots,” or mycorrhizae, are essential to the normal growth and development of forest trees and many herbaceous plants. Orchid seeds will not germinate until mycorrhizal fungi invade their cells. In virtually all of the woody trees and shrubs found in forests, the fungal threads grow between the walls of the outer cells of the cortex but rarely penetrate into the cells themselves. If they should happen to penetrate, they are apparently broken down and digested by the host plants. In herbaceous plants, the fungi do penetrate the cortex cells as far as the endodermis, but they cannot grow beyond the Casparian strips. Once inside the cells, the fungi branch repeatedly but do not break down the plasma or vacuolar membranes.

Some plants do not seem to need mycorrhizae unless there are barely enough essential elements for healthy growth present in the soil. Plants with mycorrhizae develop few root hairs compared with those growing without an associated fungus. Mycorrhizae have proved to be particularly susceptible to acid rain; this may signal major problems for our coniferous forests in the future if the problem of acid rain is not solved. Methyl bromide, used in the past to sterilize seed beds, kills all soil organisms,
including mycorrhizae; its continued use in the United States has been banned.