NOTES 
ON THE ORIGIN OF PARASITISM IN FUNGI 1 .— Up to the present no 
definite explanation has been offered as to why a given parasitic Fungus is often only 
capable of infecting one particular species of plant. This, however, is well known to 
be the case, for although the spores of Fungus-parasites germinate freely on the surface 
of any plant when moist, infection only takes place when the spores germinate on the 
particular species of plant on which the Fungus is known to be parasitic. This 
apparently selective power on the part of the Fungus I consider to be due to 
chemotaxis. 
An extensive series of experiments was conducted with various species of Fungi, 
including saprophytes, facultative parasites, and obligate parasites, and the results are 
given in tabulated form in the full paper. The chemotactic properties of substances 
occurring normally in cell-sap were alone tested ; among such may be enumerated 
saccharose, glucose, asparagin, malic acid, oxalic acid, and pectase. In those 
instances where the specific substance, or combination of substances, in the cell-sap 
assumed to be chemotactic could not be procured, the expressed juice of the plant 
was used. 
These experiments proved that saprophytes and facultative parasites are positively 
chemotactic to saccharose, and this substance alone is sufficient in most instances to 
enable the germ-tubes of facultative parasites to penetrate the tissues of a plant, unless 
prevented by the presence of a more potent negatively chemotactic or repellent sub- 
stance in the cell-sap. 
As an illustration, Boirytis cinerea , which attacks a greater number of different 
plants than any other known parasite, cannot infect apples, although saccharose is 
present, on account of the presence of malic acid, which is negatively chemotactic to 
the germ-tubes of Botrytis. 
In the case of obligate parasites the cell-sap of the host-plant proved to be the 
most marked positive chemotactic agent. Malic acid is the specific substance that 
attracts the germ-tubes of Monilia fructigena into the tissues of young apples; 
whereas the enzyme pectase performs the same function for the germ-tubes of 
Cercospora cucumis , an obligate parasite on the cucumber. 
Immune specimens of plants belonging to species that are attacked by some 
obligate parasite owe their immunity to the absence of the substance chemotactic to 
the parasite. 
Purely saprophytic Fungi can be educated to become parasitic, by sowing the 
spores on living leaves that have been injected with a substance positively chemotactic 
to the germ-tubes of the Fungus experimented with. By a similar method of pro- 
cedure, a parasitic Fungus can be induced to attack a different species of host-plant. 
1 Abstract, reprinted from the Proceedings of the Royal Society. 
[Annals of Botany, Vol. XVIII. No. LXX. April, 1904.] 
Z 
