62 Relations betit\een Host and Parasite. 



a year, but towards the end their germinating power began to fail and at last 

 failed completely. " Smut germs which have lived too long and too exclusively 

 outside of the host-plant and multiplied in the form of yeast conidia lose 

 their infective power conjointly with the ability to throw out germ-tubes.'" 



It has also to be noted that the climatic conditions which favour the para- 

 site are not always those which are most conducive to the well-being of the 

 host, especially when it is being attacked from the outside. The dull, damp, 

 cold weather which makes its tissues tender and renders its functions less 

 active are just those conditions which favour the entrance of the fungus, and 

 if the latter once reaches the growing-point, it is as certain to succeed as the 

 plant itself. Brefeld has shown this beautifully by demonstrating that if 

 the conidia are forced into the bud by means of a syringe, so that they can 

 germinate in contact with the young and growing cells, infection can always 

 be assured. 



There is, generally speaking, a double set of factors involved in each case 

 of parasitism, the internal disposition or constitution of both host and para- 

 site and the external influences or environment affecting each. It is iu the 

 mutual play and interaction of these factors that all the varied and varying 

 effects are produced, and it can therefore be readily understood that no single 

 cause or set of factors will explain any case of parasitism, but that the life- 

 history of both host and parasite must be considered and all the agencies 

 which affect one or other favorably or unfavorably must be taken into ac- 

 count. The study of the parasitic diseases of plants, or the mutual relations 

 of host and parasite, thus becomes a very complicated problem, and the recog- 

 nition of the factors acting in concert or successively which bring about the 

 changes giving rise to disease, is the only way to arrive at its true nature. 

 While it is true that no single factor is entirely responsible for a disease, yet 

 it is customary to classify diseases according to their chief causes. John 

 Stuart Mill says that " in practice, that particular condition is usually styled 

 the cause, whose share in the matter is superficially the most conspicuous." 

 In this sense the weather might be said to be the " cause " of some parasitic 

 plant diseases, as they are conspicuously associated with particular weather 

 conditions. But among the various factors concerned, that one without 

 which none of the others would be effective in producing the particular symp- 

 toms, may be more truly termed the cause. Hence when the presence of a 

 fungus, such as the smut-fungus, is absolutely essential to the production of 

 a well-marked disease, it is the direct cause of it, even although such a co- 

 operative factor or contributing cause as the weather is necessary. 



