302 DISEASES OF CULTIVATED PLANTS 



but is most general on wheat. It is also abundant on wild 

 grasses, occurring on one hundred different species in Sweden 

 alone, according to Eriksson. 



The spring condition of the fungus, known as the aecidium or 

 ' cluster-cup 'stage, grows on the leaves, flowers, and young fruit 

 of the barberry. This condition is accompanied by sperm o- 

 gonia, respecting which but little is known ; they are generally 

 considered as representing an effete male element. The 

 aecidiospores are dispersed by wind, and those that happen 

 to alight on the leaf of a wheat plant or other suitable grass, 

 germinate and the germ-tube enters the tissues, and in course 

 of time produces the uredospore or second stage in the life- 

 cycle of the fungus. Uredospores are formed in rapid 

 succession so long as the host-plant remains vigorous, and as 

 these are dispersed by various agents, and infect every wheat 

 plant they alight upon, the disease under favourable condi- 

 tions spreads rapidly, the epidemic of disease depending 

 entirely upon infection by the uredospore stage of the 

 fungus. Towards the ripening period of the host-plant the 

 pustules that had previously produced uredospores, now give 

 origin to the last phase in the life-cycle of the fungus, viz., 

 teleutospores or resting-spores, whicli remain in an unchanged 

 condition until the following spring, when they germinate 

 and produce small secondary spores, which infect barberry 

 bushes, when the cycle of development repeats itself. 

 Although the three stages mentioned enter into the entire 

 life-cycle of the fungus, they are not all absolutely essential 

 to its continuance in time. The aecidium stage may be 

 entirely omitted, and it is in the southern hemisphere and 

 on the plains of India, nevertheless the rust of wheat is as 

 rampant as ever. In localities where suitable host-plants 

 live throughout the year, the uredo stage alone is necessary 

 for the continuance of the disease. In many other species 

 of fungi having more than one stage in the life-cycle, it has 

 been proved that one or more stages may drop out entirely, 

 without arresting the development or extension of the species. 

 Dr. Eriksson who has paid considerable attention to the rust 

 of cereals, states that there are at least ten kinds of fungus 

 which cause rust on cereals, some of these are species, others, 

 are biological forms. Until recently it was assumed that 

 cereals could be infected by the rust growing on any kind o f 

 grass, Eriksson has proved that this assumption is not in 

 accordance with facts, but that many of the rusts on grasses 



