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 but are dependent upon the association for normal development. (Mutualism is a form of symbiosis; see page 300.)

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 (Fig. 5.16), are essential to the normal growth and development of forest trees and many herbaceous plants. Orchid seeds will not

Chapter 5

Chapter 5

Plant Growth And Development Flowering

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