The relation of the mycorrhizal soil fungi to the growth of conifers has long been a controversial subject.³ Recent experiments in nursery production, in afforestation and in extending the range of certain species of conifers (11, 14, 21, 32, 36, 37, 38) have reemphasized the importance of these fungi in promoting tree growth and aiding nutrition. Nonetheless, the manner in which these organisms work is still obscure, and growth stimulation of seedlings has been attributed both to effects resulting from the invasion of short roots by fungi and to the activity of the fungus in the soil adjacent to the roots.

The invasion of the short roots by the fungus commonly results in the formation of ectotrophic mycorrhizae which are characterized by an external fungal mantle and an internal Hartig net (16). When the fungus is more active, intracellular haustoria (48) are formed in addition to the intercellular net, and these types of mycorrhizae have been called ectendotrophic. The nature of the fungal infection is apparently dependent to a considerable degree upon the species of fungus and the relative vigor of both the host plant and the fungus. Endotrophic mycorrhizae, which are characterized by intracellular infection, but which lack the fungal mantle and Hartig net, are apparently not common among the pines.