1.—PHYSILOLOGY. 199 
Chodat (1913) have demonstrated that cultivated gonidia develop four 
times as well when supplied with glycocoll or peptone in place of 
potassium nitrate. 
The carbon supply of gonidia, according to Artari (1899, 1901), 
Radais (1900), and Dufrenoy (1918), is not derived photosynthetically, 
but from the substratum on which they grow. Whilst Tobler (1911), in 
his culture experiments with lichens, found that the gonidia obtain their 
carbon from calcium oxalate secreted by the fungus, Chodat (1913) 
observed that cultured gonidia grow but slowly without sugar (glucose), 
which he believes constitutes their main source of carbon supply. 
Whereas, according to Chodat, the gonidia grow poorly on organic 
nitrogen in the absence of sugar, they develop rapidly when sugar is 
added. He therefore concludes that the gonidia lead a more or less 
saprophytic life in that they obtain from the fungus-hyphe both organic 
nitrogen and carbon in the form of glucose or galactose. 
The nutrition of fungi in lichens depends partly upon parasitism, 
when they invade the gonidia, and partly upon saprophytism, when 
they utilise dead gonidia (Chodat). 
In concluding this section, the hypothesis of M. and Mme. Moreau 
(1921) demands mention, since it bears upon the manner in which 
lichens may have originated in nature. . They regard the fungal portion 
as a gall-structure arising from the action of the associated alga. The 
lichen, according to this view, is to be regarded as a fungus that has 
been attacked by a chronic disease which has become generalised and 
necessary for the subsistence of the host-fungus. F. Moreau (1922) 
sums up this view as follows: ‘The lichen-fungus appears as an 
organism characterised in its morphology by deformity due to an infec- 
tive agent, an alga. The history of the association existing in lichens 
may be described as that of a contagious malady marked by the invasion, 
development, inhibition, and death of the infective agent on the one 
hand, and on the other hand by the morphological reactions and defen- 
sive processes of the attacked organism. Im conformity with the 
virulence and relative immunity of the two opponents, the struggle may 
be short, the association transitory, the conflict may last indefinitely, 
and the association, rendered lasting, presents the appearance of a 
harmonious symbiosis.’ 
(2) The Root-nodules of Leguminous and other Plants. 
A well-known example of symbiosis is afforded by the presence of 
the bacteroids in the nodules of leguminosie, the micro-organisms being 
capable of fixing atmospheric nitrogen and thereby rendering nitrogen 
available for assimilation by the plant. This was demonstrated by 
Hellriegel and Willfahrt (1888), Schloesing and Laurent, whilst Beije- 
rinck cultivated Bacterium radicicola from the nodules and produced 
nodules synthetically by bringing the plant and bacterium together on 
previously sterilised soil. According to Pinoy (1913), the bacteroids 
are myxobacteria, and, in the case of one species which he has specially 
studied (Chondromyces crocatus), it was found essential for the 
successful cultivation of the micro-organism, apart from its host-plant 
and in vitro, that it should he grown in association with a species of 
