Tuberculate mycorrhizas of Castanopsis horneensis King and Engelhardtia roxburghiana Wall

summary Tuberculate mycorrhizas of Castanopsis borneensis King (Fagaceae) and Engelhardtia roxburghiana Wall. (Juglandaceae) were studied by light and electron microscopy. The tubercles of both trees showed similar structure and morphology. The fungal partner is an unidentified basidiomycete. Beside normally developed mycorrhizas in each tubercle some heavily‐infected roots were found. These roots are characterized by a very thick hyphal mantle around a dead root tip and a circle‐like ingrowth of the fungus up to the vascular tissue – the strangulation zone. All cells distal to the strangulation zone were dead, but basipetal to it, normal cortical tissue with an epidermal Hartig net was present. The outer cortex in this region showed wall ingrowths along the inner tangential walls and an accumulation of ER and mitochondria. The formation and function of the heavily‐infected roots is discussed.