Chapter 3 - Gaining water and nutrients: root function

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Seedlings of Eucalyptus globulus which have formed an ectomycorrhizal association with the fungus Hebeloma, whose white mycelium can be seen ramifying through the soil and forming basidiomes (toad-stools) above the soil (Photograph courtesy I. Tommerup, CSIRO Forestry and Forest Products)

A radicle may be compared with a burrowing mole, which wishes to penetrate perpendicularly into the ground. By continually moving its head from side to side, or circumnutating, he will feel a stone or other obstacle as well as any difference in the hardness of the soil, and he will turn from that side; if the earth is damper on one than the other side he will turn thitherward as a better hunting-ground. Nevertheless, after each interruption, guided by the sense of gravity, he will be able to recover his downward course and burrow to a greater depth.

(Charles Darwin, The Power of Movement in Plants, 1881).