GROWTH AND DEVELOPMENT OF MYCORRHIZAE OF SUGAR MAPLE (ACER SACCHARUM MARSH.)
Beaded rootlet growth in Acer saccharum is thought to be caused by intermittent growth due to alternately favorable and unfavorable moisture conditions during the season rather than by mycorrhizal infection. Both beaded and nonbeaded rootlets are mycorrhizal-infected, and rootlets growing in the deeper soil levels where moisture conditions do not fluctuate rapidly are nonbeaded. Differences found in the activity of rootlets growing in hummocks and depressions appeared to be related to soil moisture conditions found there. This paper describes (a) the anatomy of metacutinized root tips during dry soil conditions, and the relationship of this condition to the development of constrictions between beads; (b) the extrastelar anatomical characteristics of beaded and nonbeaded rootlets; (c) the morphology of the mycorrhizal fungus in its relationship to rootlet anatomy; (d) the extramatrical mycelium of the mycorrhizal fungus found in the soil of the rooting zone (the extramatrical mycelium is thought to arise from fungal hyphae and vesicles produced in live rootlets which are released to the soil from disintegrating rootlet cortical tissue); and (e) an hypothesis regarding the life history of this type of vesicular–arbuscular mycorrhizal fungus.