Final answer:
Mycorrhizas are symbiotic associations involving fungi and plant roots, present in about 80-90% of plant species, crucial for nutrient and water uptake, and for the survival of many trees and grasses.
Step-by-step explanation:
Mycorrhizas are present in approximately 80-90% of plant families studied, fostering critical symbiotic relationships between fungi and plant roots which is vital for the productivity of farmland. The two primary types of mycorrhizae are ectomycorrhizae, which form a sheath around the roots, and endomycorrhizae or arbuscular mycorrhizae, where the fungal mycelium is embedded within the root tissue. These symbiotic associations help plants absorb water and minerals from the soil effectively, while the plants in turn provide the fungi with carbohydrates produced through photosynthesis.
Mycorrhizal fungi are crucial as they enhance the root's surface area and effectively improve water and nutrient absorption. Using mycorrhizal fungal inoculants, which are promoted by supporters of organic agriculture, can help improve the growth and health of plants, although evidence on the effectiveness of such inoculants is varied. It is clear, however, that without these fungal partners, a significant portion of trees and grasses would not survive.