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and stumpy, with no root hairs. An extensive network
of individual hyphae or aggregated mycelial cords
(see Fig. 5.12) radiates from the surface of the root
sheath, while beneath the sheath the fungus invades
between the root cortical cellsto form a “Hartig
net.” Although the fungus is in close contact with the
root cells in this region, there is no penetration of the
host cells, hence the name of these mycorrhizas –
ectotrophic (outside-feeding) mycorrhizas (Fig. 13.7).
Because of the lack of root hairs and the encasement
of the feeder roots by a fungal sheath, virtually all the
mineral nutrients that enter the root must be channelled
through the fungus. The uptake of mineral nutrients

from soil is facilitated by the mass of fungal hyphae

that radiate into the soil and transport nutrients back

to the mycorrhizal sheath. The fungus benefits from

these associations by obtaining sugars from the plant.

Trees invest a considerable amount of photosynthate

to support the fungal network – conservatively estimated

at 10% or more of the annual photosynthetic produc-

tion of a tree.

Classic early experiments by Harley demonstrated the

transport of sugars from the plant leaves to the roots.

In these experiments the leaves of young trees were

exposed to^14 C-labeled CO 2 , and the fate of the label

was followed through the plant tissues. Label was

262 CHAPTER 13

Total biomass

(a)

7.5 g

Prunella vulgaris

(b)

4.5 g

A + B

+ C + D

0 B D

Brachypodium pinnatum

AC

(c)

5 g

A + B

+ C + D

0 ACB D

(d)

16 g Carex flacca

Fig. 13.5Total biomass (a) and the biomass of three representative plant species (b–d) grown in soil with no mycor-
rhizal fungus (0) or with four separate AM fungal species (A, B, C, D) or a combination of all four AM species. Note
that the vertical scale of each histogram is different but the largest biomass is shown in each case. (Data from van der
Heijden et al. 1998, with permission from the publisher, but only some of the plant species are shown in this figure.)