Mycorrhizas. ... Mycorrhizas are symbiotic relationships between certain fungi and the roots of plants. The fine fungal threads (called hyphae) either ensheath or penetrate the host plant's roots. The fungus helps the plant to extract nutrients and water from the soil, and also protects its host against harmful organisms.

Legumes form a unique symbiotic relationship with bacteria known as rhizobia, which they allow to infect their roots. This leads to root nodule formation where bacteria are accommodated to convert nitrogen from the air into ammonia that the plant can use for growth.

The symbiotic relationship that occurs between an orchid and a tree would be classified as commensalism. Most orchids are epiphytes, which mean that that they grow on other plants.