<P> A mycorrhiza (from Greek μυκός mykós, "fungus", and ῥίζα rhiza, "root"; pl . mycorrhizae or mycorrhizas) is a symbiotic association between a fungus and the roots of a vascular host plant . The term mycorrhiza refers to the role of the fungi in the plants' rhizosphere, its root system . Mycorrhizae play important roles in soil biology and soil chemistry . </P> <P> In a mycorrhizal association, the fungus colonizes the host plant's root tissues, either intracellularly as in arbuscular mycorrhizal fungi (AMF or AM), or extracellularly as in ectomycorrhizal fungi . The association is generally mutualistic, but in particular species or in particular circumstances, mycorrhizae may be variously pathogenic in the host plants . </P> <P> Mycorrhizal fungi form a mutualistic relationship with the roots of most plant species . In such a relationship, both the plants themselves and those parts of the roots that host the fungi, are said to be mycorrhizal . Relatively few of the mycorrhizal relationships between plant species and fungi have been examined to date, but 95% of the plant families investigated are predominantly mycorrhizal either in the sense that most of their species associate beneficially with mycorrhizae, or are absolutely dependent on mycorrhizae . The Orchidaceae are notorious as a family in which the absence of the correct mycorrhizae is fatal even to germinating seeds . </P>

Which of the following is not a function of mycorrhizae