118 



Mr. G. Massee. 



[Jan. 11, 



forces concerned in the evolution of a "biologic form," viz., 

 " specialising factors " and " generalising factors." 



Attention is also drawn to the close parallel between (1) the 

 behaviour of the fungus in the experiments in which the conidia were 

 sown on the tissues of the leaf exposed by the cut; and (2) the 

 biological facts obtaining in the class of parasitic fungi known as 

 " wound parasites " (Nedria, Pezim ivillkommii, etc.), which are able to 

 infect their hosts only through a wound. 



" On the Origin of Parasitism in Fungi." By George Massee, 

 Principal Assistant. Herbarium, Eoyal Gardens, Kew. Com- 

 municated by Sir William T. Thiselton-Dyer, K.C.M.G., 

 CLE., F.E.S. Eeeeived January 11— Read February 4, 1904. 



(Abstract.) 



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 were 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 substance 

 in the cell-sap. 



As an illustration, Botrytis 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. 



