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a complex, multicellular structure, with a central strand of conducting tissue, surroun-
ded by several other layers of cells; the finest roots are around seven hundredths of a
millimetre in diameter, but a tenth of a millimetre is more typical. A fungal hypha is
only a hundredth of a millimetre in diameter. The cost of building a root or a hypha is
proportional to the volume of tissue involved, and this depends on the square of the
radius: the hypha is ten times finer than the root and so one hundred times cheaper to
build. In simple economic terms, therefore, it pays the plant to harness fungal hyphae
to do the job of scavenging for phosphate rather than building roots for the same pur-
pose. The critical point is that it is almost exclusively phosphate that is so hard to
obtain: a root system quite adequate for obtaining water, or nitrate or potassium ions,
would be incapable of taking up enough phosphate, but if the plant grew more root,
the benefits would only be in extra phosphate.
This seems to be the underlying evolutionary rationale for both ectomycorrhizas
and VA mycorrhizas, but the former have taken the process one stage further. Fungal
hyphae are too fine to carry large volumes of material and they would therefore be
unable to supply enough water for a large plant, for example, simply because so much
greater a volume of water than of other soil resources is required. The hyphal strands
that emanate from ectomycorrhizal roots, however, are remarkably thick, consisting
of many hyphae wrapped together. Using electron microscopy ( see here ) , one can see
that in some cases these hyphae have fused together to create a large diameter chan-
nel, perhaps five to ten times larger than an individual hypha. This channel is big
enough to move water in large quantities and it seems that some ectomycorrhizal trees
do take up at least part of their water needs through mycorrhizal hyphae. This means
that much of the resource acquisition function of the root system can be taken over by
the fungus, reducing the need for an extensive root system, and this is precisely what
occurs in some conifer forests. These seem to have much less root than one would
expect; the root system is apparently reduced to the structural skeleton required to an-
chor the tree and to provide points of origin for the ramifying hyphae, and a limited
absorption capacity.
This is not true of all trees, since many of them have VA mycorrhizas, including
maples, sycamore, limes, poplars, and elms. Some trees, such as willows, are thought
to be able to form either type of mycorrhiza, but the reasons why different trees favour
different fungal partners in this way are quite unknown. It does seem that ectomycor-
rhizal trees are better able to colonize poor soils than VAM trees, and this is probably
because the former get more benefit from the more active ectomycorrhizal fungi. Of
course, there is a cost to this: the ectomycorrhizal tree probably has to give up more
of the carbon it fixes in photosynthesis than does the VAM tree and so the latter may
be at an advantage on better soils.
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