THE RUSTS OF NOV 7 A SCOTIA. FKASER. 315 



These spores always follow each other in a definite order- 

 that is, first pycniospores, then aeciospores, urediniospore-s, 

 and lastly teliospores. One or more of the spore forms may 

 be absent in certain genera or species, except the teliospores, 

 but the spore succession is the same. The basidiospores are 

 produced on a short promycelium developed directly from the 

 teliospore and are thus always present. The largest number 

 of rusts have all the spore forms present; the next largest is 

 the series with all the forms suppressed except the teliospores ; 

 then the series with the urediniospores absent, and the smallest 

 number is that in which the aeciospores are wanting. Not 

 much attention has been given to the presence or absence of 

 tihe pycniospores, for they seem t0 be present in a gpreat 

 majority of species, though wanting in a few. 



Mycelium. 



The vegetative mycelium of the rusts is very inconspicuous. 

 It is much branched, colourless, septate and usually ramifies 

 in the walls of the cells, sending haustoria into the cavities. 

 It may be localized or it may permeate the whole plant, and 

 in the latter case is often perennial. Thus the fungus may 

 appear in the same plant year after year without spore infec- 

 tion. In Puccinia obtegens, the rust of the Canada thistle, the 

 mycelium lives in the upper part of the rootstock during the 

 winter and infects the young shoots in the spring. In the 

 "'Orange Rust of Raspbeiry" (Gymnoconia interstitialis) the 

 aecial mycelium is perennial in the canes and thus lives from 

 year to year. 



Eriksson has advanced the theory that the mycelium may 

 live in the cells of the host in the form of a plasma intimately 

 mixed with the protoplasm of the host, and that under favour- 

 able conditions, as in the spring, this mycoplasm develops into 

 the ordinary mycelium. This would account for the winter- 

 ing of many rusts where the aecial hosts are not found and 

 for the sudden outbursts that often occur in the cereal rusts. 



