358 Groom . — On Thismia Aseroe ( Beccari ) 
At present the weight of evidence seems to lie on the side 
of the symbiotic view. In favour of it are Frank’s rough 
observations on forest-trees deprived of the Fungus, and the 
facts given with reference to Thismia showing that there is 
a mutual interchange of material. The distribution of the 
hyphae, however, might as readily be explained on the infec- 
tion-view as on the symbiotic theory, now that it has been 
shown, at any rate in endotrophic mycorhiza, to be as largely 
a matter of chemotropism. 
Viewing the mycorhiza of Thismia as a case of symbiosis 
between host and Fungus, it will be seen that there is one point 
of strong similarity to the symbiosis between low Algae and 
nitrogen-fixing Bacteria, and to the relation between the 
Fungus of leguminous tubercles and the host, namely, that 
there is an increased assimilation of nitrogen and manufacture 
of proteids. In his beautiful piece of work on a nitrogen-fixing 
Bacterium, Winogradski showed that the amount of nitrogen 
fixed was proportional to the amount of dextrose supplied. 
Transferring this idea to the case in which Bacteria and Algae 
together fix free nitrogen, it seems fair to conclude that the 
Alga supplies the carbohydrate. Again, leguminous plants 
may be cultivated in ordinary water-cultures and tubercles 
develop. Here the carbohydrate is indubitably supplied by 
the host, and Professor Vines’ experiments showing that 
adequate illumination was essential to ensure the develop- 
ment of tubercles are thus explicable 1 . With regard to 
ectotrophic mycorhiza on green plants, there is no evidence 
that the Fungus absorbs carbohydrates from the host, though 
there is the collateral evidence in green Orchids that in endo- 
trophic mycorhiza carbohydrates are absorbed (cp. remarks on 
Thismia). It is easy to comprehend the advantage of this 
arrangement accruing to a mycorhizal Fungus on a green 
plant which can assimilate carbon dioxide. But does the 
1 Professor Vines informs me that he has found that when Beans are germinated 
and grown in the dark, tubercles are developed on the roots of the seedlings : it 
would appear that, in this case, the necessary carbohydrate is supplied from the 
reserves in the cotyledons. 
