There are two primary types of mycorrhizal fungi, endo and ecto, both are mycelium but associate with different types of plants while performing similar functions.
Endomycorrhizae (or Vesicular Arbuscular Mycorrhizae) are symbiotically associated with about 90% of the plant kingdom. This coevolution began 350–460 million years ago and allowed vascular plants to begin growing on land. Endomycorrhizae physically penetrate plant roots and create an intercellular attachment in the form of branched structures called arbuscules. Arbuscules within root cells provide an extensive surface area for the exchange of nutrients such as glucose or nitrogen through the cell membrane. Mycelia from endomycorrhizae extend from the plant roots into the surrounding soil, gathering nutrients and water bringing them back to the plant’s roots.
Ectomycorrhizae only symbiotically associate with about 5% of the plant Kingdom. This includes conifers and a few deciduous hardwood trees. They form an extracellular attachment by creating a fungal sheath around the root tips of the tree. In this case, the fungus wraps around the plant root rather than penetrating the cells. The fungus excretes organic chemicals that cause the root cells to become more permeable allowing for greater exchange of gas, water and nutrients. The mycelium of these fungi forage deep within the soil and transport nutrients and water back to the root tips of the tree.