1002.] Starvation on the Parasitism of Puccinia, 151 



causes at work in the living cell which confer immunity or predisposition 

 on the species of host-plant, or which confer virulence or impotence on 

 the spore, they lie deeper than nutrition, reminding us once more of 

 the significant resemblances which, as I pointed out in a previous 

 paper,* exist between the phenomena of infection and those of polli- 

 nation. It is in the highest degree improbable that the pollen-tube of 

 a given species, A, is incapable of growth in the style and the ovule of 

 an allied species, B, simply because the tissues' of B do not contain 

 suitable food-materials, while the pollen-tube of a species, C, readily 

 fertilises a more distant species, D, simply because the latter does con- 

 tain suitable nutritive materials, especially as in both cases we may be 

 able to germinate such pollen in artificial sugar-solutions. 



All the evidence points to the existence, in the cells of the fungus, 

 of enzymes or toxins, or both, and in the cells of the host-plant of 

 anti-toxins or similar substances, as the decisive factors in infection or 

 immunity, although I have as yet failed to isolate any such bodies. 



Moreover, I regard the results here given as furnishing strong evi- 

 dence, on the whole, against any hypothesis which assumes the exist- 

 ence of a latent or lurking source of disease in the plants themselves, 

 and as supporting the view that every patch of pustules originates 

 from a definite infection spot due to the entrance of a germ-tube from 

 a spore which has there germinated on the leaf. 



* 'Proc. Camb. Phil. Soc.,' vol. 11, parfc 5, p. 307. 



VOL. LXXI. 



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