ROOTS - The Soil - page 53

successfully germinate if they can associate with their particular mycorrhizal fungus.
In their early growth they establish a tuber-like structure called a protocorm, and at
this stage they are almost totally dependent on their fungal partner, which supplies
them not only with mineral nutrients, but also with carbon for energy, for the orchid
has not yet started to photosynthesize. Later, most orchids develop green leaves above
ground and the relationship then comes to resemble other types of mycorrhizal asso-
ciation.
Some orchids, however, never produce green shoots, have no chlorophyll and
cannot photosynthesize. These non-green orchids are particularly characteristic of
very deep shade in forests, where there is insufficient light for green plants to survive.
The bird's-nest orchid Neottia nidus-avis is the commonest, but others include the
ghost orchid Epipogium aphyllum ( Plate 5 ) and the coralroot orchid Corallorhiza tri-
fida. These are normally described as being saprophytic, feeding on decaying organ-
ic matter, which is certainly unusual behaviour for a plant. The truth is even more
bizarre. The root systems of these orchids are extremely limited, consisting usually of
a short stubby rhizome, or a tangled knot of thick roots (the eponymous 'bird's nest').
They therefore present the unusual situation of a plant effectively without leaves or
roots! Other plants like this, such as broomrapes, are parasites, and these orchids are
no exception, but whereas broomrapes are parasitic on other plants, the non-green
orchids parasitize their mycorrhizal fungi, which supply them with all their needs.
This has almost certainly arisen as an extension into adult life of the juvenile depend-
ence of all orchids, but it is certainly remarkable that an immobile plant with virtually
no roots and no leaves can parasitize a fungus which is free to grow through soil and
may itself in some cases be parasitizing other green plants.
Some other plants can perform the same trick, especially the yellow bird's-nest
Monotropa. This associates with the same fungi that are mycorrhizal with forest trees,
and so parasitizes those trees indirectly. By feeding pine trees with radioactive carbon
dioxide, which they fix into sugars, it is possible to trace the path of the carbon first
into the pine's mycorrhizal partner, the fungus Boletus , and then into the Monotropa.
Such fungal connections between plants are not unusual, as we have seen already.
Most plants are mycorrhizal, though the types of mycorrhiza and their functions
vary widely. As a result there has been great interest in the possibility of using mycor-
rhizas commercially, but with the exception of commercial forestry, where the trees
become infected willy-nilly in most cases, and of specialized horticulture (particularly
of orchids), the applications have been disappointing. In most agricultural situations
it is cheaper to apply phosphate fertilizer than to inoculate plants with mycorrhizal
fungi, and only where the soil has been sterilized to eliminate pathogens, as in citrus
orchards in Florida, have the benefits been realized.