102 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”colonize the roots of some plants (notably the legumes such as soybeans),
forming root nodules where the bacteria provide the plant with nitro-
gen “captured” from the air while the roots serves as a carbon source
for the bacteria. Beneficial fungi that live symbiotically with the roots
help transport water and minerals (notably phosphorus) to the roots,
while taking in carbohydrates from the plants. About 80% of land plants
establish mycorrhizal relationship with fungi. The fungal mycelia form a
network around a tree and increase the surface absorbing area of roots
by 100 to 1,000 times (Fig. 5.4). When roots from two or more plants
coexist with the same fungal mycelia, a symbiotic network is formed,
which greatly expands the underground communication among plants.
This network serves as a conduit not only for nutrition but also for trans-
mission of stress signals. A large underground fungal network increases
leaf nitrogen content and reduces the incidence of pathogen-induced
root lesions; this beneficial effect is more prominent if the plants are
Fig. 5.4. Colonization of plant roots by symbiotic mycorrhizal fungi. The left two
plants are grown without the fungi; the right two plants are grown in the presence of
the fungi, depicting a continuous network formed by the roots and the fungal mycelia.