POLYMORPHISM. 



201 



some winter rye was sown round a berberry bush, which in the 

 following year was infested with Mcidium, which was mature in 

 the middle of May, when the rye was completely covered with 

 rust. Of the wild grasses near the bush, Triticum repens was 

 most affected. The distant plants of rye were free from rust. 



The spores of the Mcidium would nofc germinate on berberry 

 leaves ; the berberry Mcidium could not therefore spring from 

 the previous Mcidium. The uredospores of Puccinia graminis 

 on germinating penetrate into the parenchym of the grass on 

 which they are sown ; but on berberry leaves, if the tips of the 

 threads enter for a short distance into the stomates their growth 

 at once ceases, and the leaves remain free from parasites. 



FIG. 107. Cells and pseudospores of JEcidium berberidis. 



Montagne has, however, described a Puccinia berberidis on 

 leaves of Berber is glauca from Chili, which grows in company 



FIG. 108. Cells and pseudospores of JEcidium graveolens. 



with Mcidium berberidis. This at first sight seems to contradict 

 the above conclusions ; but the Mcidium which from the same 

 disc produces the puccinoid resting spores, appears to be dif- 

 ferent from the European species, inasmuch as the cells of the 

 wall of the sporangium are twice as large, and the spores de- 

 cidedly of greater diameter.* The resting spores, moreover, 



* We have before us an Mcidium on leaves of Berberis vulgaris, collected at 

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