199 



may be infected. On the other hand, uredospores from other 

 collateral hosts fail to infect the wheat, except in rare cases 

 under very favourable conditions. It has in fact been clearly 

 established that this wheat rust is a highly specialized form of 

 the fungus which possesses very little capability of directly 

 passing on to and infecting other nearly allied" species of 

 plants.* Another suggestion is that the uredospores may live in 

 the soil during the period when wheat is out of the ground 

 (between April and November) and be able to infect the young 

 crop in December January. Experiments, however, with 

 allied fungi have shown that the uredospores soon lose their 

 power of germination, especially when they are exposed to a 

 high temperature, as they would be in the hot season in the 

 plains of India. 



These explanations therefore can hardly account for the per- 

 sistence of the disease. Again it has been suggested that the 

 sporidia may be able to give rise to a mycelium in the wheat 

 plant capable of producing uredo- and teleuto-spores. This is 

 improbable and there is no evidence to support it. The most 

 recent researches on the subject indicate that the rust on wheat 

 may be a truly hereditary disease. The protoplasm of the 

 fungus is said to be able to exist in intimate union with the 

 protoplasm of the wheat plant, and its presence then can only 

 be detected with great difficulty. This state of things may 

 continue for months, or even years, the disease germs passing 

 on from generation to generation in the seed, and it is only 

 under certain favourable conditions of moisture, heat, and of the 

 state of the host-plant itself, that the fungus becomes distinctly 

 visible in the form of a mycelium, which takes place just before 

 the rust pustules begin to appear. 



The only practical measures at present available for com- Preventive 

 bating this fungus consist in selecting the most rust-resistant Measur ^ s> 

 varieties of wheat for continued cultivation and in endeavour- 

 ing to produce new and improved varieties by inter-crossing. 



* Seeing that the various forms of this fungus, occurring on different collateral 

 hosts, are each more or less confined to some special species of plant we cannot 

 doubt that in some way they differ essentially from one another. At the same 

 time, so far as visible microscopic characters are concerned, these forms cannot be 

 distinguished and hence they are all grouped under the species Puccinia graminis. 

 Here we appear to have an example of the power possessed by some plants of 

 adapting themselves to changed conditions, and we see that, having become 

 accustomed to one set of conditions, they do not readily adapt themselves to 

 another, although hi time a specialized form may arise which can do so. 



