untitled

Fig. 13.6The fruitbodies (basidio-

carps) of some representative fungi

that form ectomycorrhizas with forest

trees. (a) Hebelomasp. growing on the

roots of an oak seedling in a plantpot.

(b) Amanita muscariagrowing on pine

roots in a field site. (c) Lactariussp.

(which, typical of the genus, secretes a

milky fluid when the gills are dam-

aged) growing on birch roots.

(a) (b)

(c)

found in the form of common plant sugars such as

sucrose and glucose in the stems and root tissues, but

when the label entered the fungal sheath it was found

in the form of^14 C-labeled sugar alcohols such as man-

nitol, arabitol, and erythritol. Thus, there is effectively

a one-way flow of carbohydrates from the plant to the

fungus, because most plants cannot utilize these

“fungal sugars” (Harley & Smith 1983; Harley 1989).

Many ectomycorrhizal fungi can grow in laboratory

culture on simple organic media, but they have little

or no ability to degrade cellulose and lignin, unlike

the common decomposer fungi of the Ascomycota

FUNGAL SYMBIOSIS 263