determine that the teleutospores can germinate upon corn 

 plants and grasses, and whether the climatic conditions which 

 differ totally from those of other rust-affected countries shorten 

 the chain of stages in the life of the fungus believed to be 

 necessary in Europe. 



Attention is also being directed to this question by mycolo- 

 gists in all parts of the world since the Australasian Rust Con- 

 ferences have made fully public the extraordinary prevalence of 

 rust in hot, arid regions, and of its persistence without the bar- 

 berry. 



PREDISPOSITION. 



A certain predisposition of the host plant of parasitic fungi, 

 or of many of them, is generally recognised. In some cases an 

 unhealthy state of a plant may make it liable to receive injury, 

 and this unhealthy state may be caused by its environment, 

 such as influences of weather, situation, soil, cultivation, nitro- 

 genous manurings, changes of temperature, excess, or lack, of 

 water, and by conditions causing chemical changes in the 

 cellular system of the plant. De Bary says, " The physiological 

 reason for these predispositions cannot in most cases be exactly 

 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 the food. In the case of the 

 Pythieae, for example, it is easy to see that the host displays 

 degrees of susceptibility or power of resistance in presence of 

 the parasite proportioned to the amount of water which it 

 contains."* 



Mr. Marshall Ward also deals ably with this subject, and 

 shows that the host plants of fungi are more or less predisposed 

 to receive injury according to the limits of their health.f After 

 describing certain chemical and structural changes produced by 

 weather and other influences, Mr. Marshall Ward concludes 

 that " under certain circumstances the parenchymatous tissues 

 of the living plant may be in a peculiarly tender, watery con- 

 dition where the cell walls are thinner and softer, the proto- 

 plasm is more permeable and less resistant, and the cell sap 

 contains a larger proportion of organic acids, glucose and soluble 

 nitrogenous materials than usual. When the external conditions 

 become more favourable the temperature higher, the air drier, 

 and the sunlight more powerful increased transpiration and 

 respiration lead to more normal metabolic activity, for which 

 energetic assimilation provides the materials. Of course, all 

 kinds of combinations are possible in detail, but when dull, 

 cold, wet weather prevails for some time, after a period of bright, 



De Bary. Op. cit. 



j- Sorauer also holds that predisposition depends upon the limit of 

 health" Breite der Gesundheit," in his work, Die Schaden der e'nlieimischen 

 Kulturpflanncn, 1888. 



