Predisposition. 61 



In 1894 several rhizomes of Peltandra virginica were sent to him from 

 North America, and planted in pots. An aecidium developed on the sterns 

 and midribs of the leaves, which was found to be new and named Aecid- 

 ium importation. One plant was left in the pot, which was placed in 

 water, and the others were planted out in a soil composed of damp humus. 

 The pot plant has annually produced the aecidia up till the time of writing 

 (1902), while the other plaints only showed the fungus very slightly in 

 1895 and 1896, after which they grew exceedingly strong, and since then 

 have remained perfectly sound. It must be remembered that this is a 

 hardy marsh-loving plant, and there is probably more than mere nutrition 

 concerned in its freedom from disease, since it would be more reasonable to 

 regard the result mentioned as due to the change of situation than to change 

 of soil. Salmon 3 considers that the evidence which is gradually accumulat- 

 ing on the subject of the relations between host-plants and parasitic fungi 

 leads us to the conclusion that immunity and susceptibility are due to con- 

 stitutional (physiological) peculiarities, and not to any structural ones. He 

 has also shown experimentally that while the uninjured leaf may be im- 

 mune, the same leaf when cut or injured may /become liable to infection, 

 and the conidia produced on such leaves are then able to infect uninjured 

 leaves. In this way the range of infection of a biologic form may be in- 

 creased. Different species of the same genus, when they are generally at- 

 tacked by a rust-fungus, may vary considerably in their susceptibility to 

 infection. 



If we attempt to explain the varying susceptibility of different plants 

 or different kinds of plants, then the difficulty is apparent, and the sym- 

 biotic relation between the parasite on the one hand, and the host-plant on 

 the other, complicates the matter. Why is the fungus able to infect certain 

 host-plants, and not others? Why is the host-plant capable of resisting 

 certain fungi, and not others? How is the fungus able to accommodate 

 itself to certain plants, and not to others ? These and other questions may 

 be asked, but cannot be fully answered at present. De Bary 2 says : "The 

 physiological reason for these predispositions cannot, in most cases, be ex- 

 actly stated ; but it may be said in general terms to lie in the material 

 composition of the host, and therefore to be indirectly dependent on the 

 nature of its food." 



The question has been asked, if there is any relation between liability 

 to infection or power of resistance and the visible structural features of the 

 leaf, and it has been answered differently by various investigators. 



Hennings 2 lays stress upon the physical characteristics of the parts of 

 the plant on which the parasitic fungi occur. He considers that the para- 

 site develops differently on a thin-skinned, delicate leaf, and a thick- 

 skinned, firm, leathery 'leaf. Also that the venation and hairiness of the 

 leaf may affect the result. 



Marshall Ward 1 fully investigated the structural peculiarities of the 

 leaves of the various species of Bromus used in his infection experiments 

 with brown rust such as thickness of cell-wall and cuticle, " bloom," 

 size, number, and distribution of hairs, distribution of chlorophyll-tissue 

 and vascular bundles and he arrived at the conclusion that " the resistance 

 to infection of the immune or partially immune species and varieties is 

 not to be referred to observable anatomical or structural peculiarities, but 

 to internal, that is, intra-protoplasmic properties beyond the reach of the 

 microscope. ' ' 



Salmon 3 comes to the same conclusion from his infection experiments 

 with the oidium of Erysiphe, but the physical characters of the wheat-plant 

 seem to have some some effect on its liability to rust, for there are certain 



