394 CLASS BASIDIOMYCETEAE 



more germ pores) and more or less colored, from light yellow-brown to 

 almost black. In some of the rusts on ferns they are colorless and thin- 

 walled. 



From the mature uninucleate teliospore the promycelium emerges at 

 once (various species of Cronartium, Puccinia malvacearum, etc.) or only 

 after over-wintering {P. graminis Pers.) or after certain unfavorable en- 

 vironmental conditions have passed. It emerges through one of the germ 

 pores and into it may pass the undivided nucleus of the teliospore or the 

 first of the two divisions of the nucleus may occur before the nuclei enter 

 the promycelium. The final division usually takes place in the promyce- 

 lium whereupon septa are formed dividing it into four cells. Sometimes a 

 further septum cuts off the teliospore from the basal cell of the pro- 

 mycelium. From the promycelial cells sterigmata grow bearing at their 

 tips the sporidia which are shot off violently at maturity. In the case of a 

 compound teliospore, such as is characteristic of Puccinia, Phragmidium, 

 Ravenelia, etc., each of the component cells produces its own mycelium, 

 thus demonstrating that each cell is to be considered a teliospore, not the 

 whole compound structure. 



Rusts may be either autoecious or heteroecious. In the former the 

 sporidial and aeciosporic infections take place on the same or closely 

 related species of host while in a heteroecious species the host infected 

 by the sporidia belongs to a family not at all closely related to that con- 

 taining the host for the aeciosporic infection. Heteroecism was first proved 

 by actual inoculation experiments by de Bary in 1865 for Puccinia 

 graminis and by Oersted for Gymnosporangium sabinae (Dicks.) Wint. in 

 the same year. The former showed that the aeciospores produced on the 

 common barberry {Berheris vulgaris L.) would not infect that species but 

 would infect the small grains such as wheat, barley, etc. The urediospores 

 would infect the same and often also closely related species of grains. The 

 sporidia from the teliospores on the overwintered straw or stubble would, 

 on the contrary, infect only the barberry. Thus was brought the scientific 

 explanation of a phenomenon known for a hundred years or more that the 

 presence of barberry plants was detrimental to small grains. This much 

 had been definitely proved by careful observation and experiment long 

 before, but the actual connection of the rust on barberry with that on the 

 grain was left for de Bary to prove. Plowright (1889) gives an excellent 

 account of these early observations and beliefs as to the harmful effect of 

 barberry on grain. Since that time the heteroecism of hundreds of species 

 has been demonstrated. It is worthy of note that Waterhouse (1929) 

 discovered one physiologic race of wheat rust that is incapable of infecting 

 barberry. 



In an autoecious species the aeciospores infect the same or closely 

 related host species as do the sporidia. Thus Pi/cama helianthi Schw., the 



