196' THE AUSTRALIAN NATURALIST. 
ous food and in return receives watar and mineral food. Still 
the arrangement is rather one-sided, as the alga can, and does, 
live independently, whilst the fungus is dependent for life 
upon the algae being entangled in the meshes of its hyphae. 
This association of green plants and fungi introduces the sub- 
ject of Mycorhizae. : 
Iycorhiza is the term applied to the symbiotic association 
of flowering plants (usually) with fungus mycelia, which live 
in or on their rootlets. Nodules are not formed. Mycor- 
hizae may be ectotrophic, the fungus forming a more or less 
interwoven feltwork over the rootlets, or endotrophic, living 
in the cortical tissues. 
In some cases, as in the Birds Nest Orchid of Europe, no 
chlorophyll is formed, though the plant is not a parasite, but 
receives its carbonaceous food from humus by means of, the 
mycorhiza on its roots. Usually the host plant is able, under 
favourable conditions, to maintain itself normally, but as the 
conditions become unfavourable, so does it become mycotro- 
phic. The mycotrophic condition enables them to utilise or- 
ganic and mineral soil food not directly available to the plants 
until made so by the action of the fungi on their roots. 
For some time it was thought that mycorhizue were as limit- 
ed in their range as the symbiotic association of Pseudomonas 
radicicola. It is now known to be more‘associated with cer- 
tain environmental conditions than any specific groups of 
plants. Scott Elliott says that almost all trees, numbers of 
shrubs, and flowering plans, ferns and their prothallia, pos- 
sess mycorhizae, but that in the case of liverworts and mosses, 
it is uncertain whether the fungus is useful or simply para- 
sitic. 
C. T. Musson records 25 species of native plants bear- 
ing mycorhizae, and states that in no plant examined were 
the roots found free from the fungi. 
Stahl has found that typically mycotrophic plants have a 
relatively low transpiration and consequently a low absorption 
of mineral food. The association is especially characteristic 
of plants growng in soils subject to drought or poor in mineral 
salts or rich in humus. Mycotrophic plants are usually of 
slow growth, with feeble transpiration and limited root de- 
velopment, their leaves rarely contain starch, and the plants 
are particularly free from mineral’ salts. Many pines and 
orchids cannot be raised from seedlings if grown in sterilised 
soil. The presence of Mycorhiza is necessary to their healthy 
development. 
It must be appreciated that undisorganised organic matter 
is quite unavailable to non-saprophytic plants. | Where con- 
