Heteroecism. 



This course of development may be represented graphically as follows, 

 taking P. gr ambus as the type: 



, in Europe. &, in Australia. 



Spennatui Teleutospores 



Teleutos pores 



Uredospores 



Aecidiospores 



FIG. 14. 



Uredospores 

 Fio. 15. 



The aecidial stage of the spring rusts of wheat (P. tritidna) or barley 

 (P. simplex] is not known, but Klebahn hazards the suggestion that the hosts 

 to which they respectively belong may only exist in their original home, and 

 thus not hitherto observed. With such widely and extensively cultivated 

 plants, the uredospores could easily be carried on the grain or by the wind, 

 and the fungus could thus be perpetuated without the intervention of an 

 intermediate host. 



The tiding over of the winter is most important for those fungi which 

 depend upon two host plants for their continued existence, but while this 

 is usually accomplished by means of the teleutospores, other and additional 

 measures may be taken .to secure the same result. Sometimes the uredo- 

 mycelium persists during the winter, and in the case of Pucdnia arrhenatheri 

 the aecidium-mycelium becomes perennial, and reproduces the aecid'ia year 

 after year, while the teleutospore is also produced. When the aecidiai 

 stage is dropped, as in the case of Pucdnia graminis in Australia, then there 

 is a profuse development of uredospores in comparison with teleutospores, 

 and there is abundance forf present needs, as well as for future germination 

 in the spring. In other cases where the aecidial host is absent, the same 

 thing has been observed, as in Caleosporium senedonis, when occurring in a 

 district destitute of fir trees, or Chrysomyxa rhododendri when the silver fir 

 is absent. 



Heteroecism ds said to increase the vigour of the fungus, and a striking 

 illustration is given by Pucdnia graminis on wheat. It is sometimes said 

 that the aecidiospores from barberry are much more virulent than the 

 uredosppres derived from the wheat itself, since the teleutospores produced 

 are earlier, more copious, and more injurious to the wheat. With reference 

 to this, Plowright says : " There is a wonderful difference in the amount 

 of injury done by mildew, when derived directly from the barberry, and 

 when derived from uredo that has reproduced itself through several gener- 

 ations. . . . The fungus grows with such energy that it so injures the 

 \vheat plant as to prevent it producing more than a few starved kernels." 

 As against this view, it may* be well to bear in mind that in no country 

 in the \yorld probably does P. graminis cause as great injury to wheat as 

 it does in Australia, a country with barberries practically non-existent, amd 

 in which the aecidial stage has never been found. 



Of course further observations on a number of species are necessary 

 :abhsh the fact, but Klebahn considers that the utilization of the 

 vegetative periods and other peculiarities of the host-plants, rendered pos- 

 sible by the change of hosts, gives the fungus a decided advantage. 



