6 BRITISH FUNGI 



leaves or leaf-sheath of wlieat or other grasses in due course give 

 origin to the conidial or summer stage of the fungus, which appears 

 under the form of short orange streaks on the leaves, etc. The 

 first-formed summer-spores, scattered by wind, etc., infect neigh- 

 bouring plants, and as tlie production of summer-spores continues 

 throughout the growing period of the host-plant, that is, the plant 

 upon which the fungus is parasitic, it can be readily understood 

 how quickly, under conditions favourable for the fungus, the 

 disease spreads and becomes epidemic, when the host-plants are 

 crowded together, as in a field of wheat. When the wheat is com- 

 mencing to ripen, summer-spores cease to be formed, and the same 

 mycelium that has produced them hitherto, now produces winter- 

 spores, or resting-spores, which remain firmly attached to the leaves 

 and straw until the following spring, when they germinate and 

 infect barberry leaves, and the cycle of development, as described 

 above, is repeated. 



The above describes the fullest development of wheat rust, but 

 under certain conditions, one or other of these phases in the life- 

 cycle of the fungus may be dropped or omitted, without in any way 

 interfering with the appearance or development of the remaining 

 stages. For instance, wheat rust in some way accompanied its 

 host-plant into the Southern hemisphere, and at the present 

 day is much more injurious to the wheat crop in Australia tlian it is 

 in Europe. But it so happens that for some reason, climatic, or 

 the absence of a suitable host-plant, the aecidium, or " cluster-cup " 

 condition of the fungus was completely omitted, so that the uredo 

 or summer form, and the telentospore or winter form of fruit are 

 alone produced in Australia. Again, where the wheat rust lias been 

 introduced into sub-tropical or warm temperate regions, where the 

 host-plant is growing all the year round, the uredo or summer form 

 of fruit is often alone produced, as there is an unbroken continuity 

 of proper host-plants for favouring the continuous growth of the 

 summer form of fruit. 



At this point the question that naturally suggests itself is, 

 why all these changes of mode of life in one kind of fungus ? At 

 present no one can offer a full explanation, yet much is known 

 bearing on this subject. A celebrated German scientist named 

 Klebs has proved, as the result of a long series of experiments, that 

 in the case of many fungi, a perpetually sterile condition, or a 

 constant production of the conidial form of fruit, or of the sexual 

 form of fruit, can be produced at will, the most important deter- 

 mining factors being the particular composition, and proportion 

 of the food supplied, along with certain physical conditions, as the 

 density of the medium in which the fungus is growing, etc. Now 

 we have alread}' learned that when a wheat plant is infected with 

 wheat rust, uredo or summer-spores continue to be produced in 

 rapid succession so long as the host-plant is still growing and 



