UREUINACEAE 291 



filled with golden yellow spores, called aecidiospores. At 

 this stage the fungus no longer continues to grow on the 

 barberry plant. The ripe aecidiospores are scattered by 

 wind, and those that happen to alight on young wheat leaves 

 or culms germinate and enter the tissues, and within a week 

 or two the third form of fruit, called the uredo stage, 

 appears on the surface of the leaves or culm, under the form 

 of brown streaks, consisting of myriads of minute brown 

 uredospores. This condition is commonly known as the 

 summer stage, as the spread of disease usually entirely 

 depends on uredospores, which are produced in rapid 

 succession during the summer months, and being readily 

 diffused, infect others plants, until frequently the entire crop 

 of wheat is infected. Towards the autumn, when the host- 

 plant is on the wane, the mycelium that had previously pro- 

 duced uredospores, now gives origin to the fourth and last 

 form of fruit, known as the teleutospore stage, known also as 

 the winter or resting-spore stage, because teleutospores 

 require to remain in a passive or resting condition before 

 they are capable of germination. In the spring following 

 their production, teleutospores germinate and give origin to 

 much smaller secondary spores. Such of these secondary 

 spores as happen to alight on young barberry leaves give 

 origin to the aecidium fruit in turn, and thus the cycle of 

 development continues from year to year. 



When heteroecism was discovered, it was assumed that 

 the destructive wheat rust, and some other parasites requiring 

 two different hosts for their complete development, could be 

 exterminated by entirely removing from the vicinity one of 

 the host-plants. This surmise unfortunately did not prove 

 to be correct, and it has since been proved that it is not 

 absolutely necessary for the maintenance of a species in 

 time, that it should pass through the entire cycle of its 

 development. In the case of wheat it is known that the 

 aecidium stage can be dropped without checking the 

 development of the parasite. In fact the disease is most 

 injurious in Australia, India, etc., where the aecidium stage 

 is unknown. In many species of rusts one or other of the 

 four forms enumerated above may be missing. The spermo- 

 gonia stage is unimportant, being at the present day simply 

 a rudiment of bygone times. In other cases, aecidium, 

 uredospore, or teleutospore phase may alone survive, or any 

 two of the three may be present in a species. In some 



