150 



Prof. KL Marshall Ward. Effect of Mineral [Nov. 4, 



treatment of the seedlings, on the leaves of the seedlings of this series, 

 similarly starved or otherwise. 



As we have seen (Experiment series No. 47, Table I, and series 60, 

 Table II), spores from a seedling starved of phosphorus, however few 

 in number, are perfectly capable of infecting the leaf of a normal seed- 

 ling. Can such spores also infect another seedling similarly starved of 

 phosphorus; and can spores reared on calcium-starved or nitrogen- 

 starved seedlings, &c, infect seedlings similarly starved of calcium or of 

 nitrogen respectively, and so on ? 



On August 18, therefore, I infected the third leaf of each of several 

 seedlings in each beaker of series 55 (Table IV) with the spores 

 developed on the corresponding seedlings (which had been similarly 

 treated) of series 50 (Table III). 



The results are summarised in Table IV. 



As the table shows, practically all the infections were successful, 

 showing that not only does mineral starvation not prevent the develop- 

 ment of virulent spores on the seedling so starved — if the latter is 

 inoculated with normal spores — but such starvation is also incapable of 

 incapacitating the corresponding seedling for infection by means of spores 

 grown on similarly starved seedlings. 



We must therefore conclude for the present that (1) the starvation 

 of mineral food-substances, although it reduces the size of the host- 

 plant and seriously diminishes the quantity of spores which the myce- 

 lium can give rise to on its leaves, does not affect either the virulence 

 of such spores or the predisposition to infection of the leaves of the 

 Brome concerned. 



Moreover (2), in view of the results with the highly-manured seed- 

 lings to which horse-dung decoction or normal mineral solution was 

 added, it seems hopeless to expect that high cultivation of this kind 

 will diminish the predisposition of the plant to infection — or, what would 

 amount to the same thing in practice — increase its resistance or confer 

 immunity, 



The effects of manurial treatment are clearly quantitative only, so 

 far as this question is concerned. 



If the host-plant is highly fed, its tissues yield more food materials 

 for the fungus ; the latter can develope a larger mycelium, and pro- 

 duce a larger crop of spores. But so long as the host-plant is capable 

 of living at all, it is a perfectly satisfactory prey for the fungus in its 

 tissues, so far as quality of fungus food is concerned. 



It seems to me that these results throw some new light on the prob 

 lem of infection and parasitism, in so far as they bear out the view 

 that the Uredine mycelium taxes the leaf — robs it of a share of its 

 food-supplies — rather than destroys the protoplasmic machinery, at any 

 rate during the vigorous period of growth and of production of Ureclo- 

 spores ; and also in so far as they suggest that whatever may be the 



