40 
The Queensland Naturalist 
June, 1939 
Dr. McLuckie, of Sydney, has compared those of Podo- 
carpus elata and Casuarina Cunninghamiana with Ps. 
radicicola, but does not definitely assign them to that 
species, though in view of the host specialisation in the 
bacteria lumped together under this name, such a diagnosis 
would not be reckless. In Cycads there are curious coral- 
loid roots at about ground level, and these are regarded 
by Spratt as receiving the initial stimulus for their forma- 
tion from Ps. radicicola, though later infection is effected 
by other soil organisms — species of Azotobacter (another 
nitrogen fixer) and the alga Anabaena cycadcarum, which 
forms a definite algal zone. 
Bacterial galls on roots are not necessarily an indica- 
tion of mutualistic symbiosis. There is, for example, the 
very destructive disease, crown gall, caused by Ps. 
tumefaciens. None of these purely parasitic bacteria are, 
however, as universally present on any species or group of 
species as those symbionts of the Ps. radicicola type, on 
whose presence the host plants have come to rely to a 
greater or less degree. 
An equally important root partnership is struck with 
fungi. Here again there are all degrees of fungal associa- 
tion with roots ranging from violent parasitism to a be- 
nign infection from which the host plant derives benefit 
which is reflected in its improved general health. Such a 
root-fungus association is known as a mycorrhiza. The 
mycorrliizal habit is well known in orchids, Ericaceae, 
Epacridaceae, Rutaceae, pines, and numerous other plants. 
The most diverse views are held on the physiological 
significance of mycorrhizas. Some consider that they re- 
present a mild and comparatively harmless parasitism, 
others that the fungi are able to exploit organic matter in 
the soil and hand it over, though not passively, to their 
symbionts, and that both partners benefit by their associa- 
tion. Evidence has accumulated to show that these fungi 
are markedly stimulated by their hosts, from which they 
obtain such food materials as carbohydrates, and that on 
the other hand the fungi are able to exploit sources of 
nitrogen not readily available to green plants. Such a 
partnership is specially valuable in soils deficient in 
nitrates, such as acid humus soils. 
Though orchid seeds can be raised aseptically, that 
is without any microbial partner, the presence of a fungal 
symbiont is necessary in nature. The existence of numer- 
ous orchids with no chlorophyll, and therefore dependent 
on ready-made organic matter, suggests in their case an 
