UREDINALES 



567 



(Strelin, 1912). The close agreement which has been found in this 

 respect between the manner of formation of spore and of peridium of the 

 aecidia and uredinia is suggestive. In Uromyces Glycerrhizae, plas- 

 mogamy appears to occur somewhere on the mycelium, as the hyphae 

 which form the uredinium are already binucleate (Olive, 1913). 



These primary uredinia which have arisen on the uninucleate mycelium 

 often differ in their appearance from the secondary ones formed later on 

 the binucleate mycelium, e.g., they are distinguished by a greater size 

 and a consequent greater deformation of the host or by a somewhat 

 different color. Because of the close morphological relationship, it is 

 often impossible to draw a sharp line between aecia and primary uredinia; 

 thus the first sorus of Phragmidium violaceum is designated by some 

 authors as an aecium (caeoma) and by others as a primary uredinium. 



Fig. 381.- 



-Triphragmium Ulmariae. Types of cell fusion in the primary uredinium. 

 (X 1,720; after Lindfors, 1924.) 



The difficulties are in part due to the fact that urediniospores some- 

 times greatly resemble the aeciospores. They are 15 to 40 /z in diameter, 

 unicellular, ovoid to cuneate, and generally finely echinulate or verrucoses, 

 the membrane is generally fairly thick, hyaline to yellow brown, and 

 pierced by several equatorial germ pores which are filled with a substance 

 of variable composition. The number and position of the germ pores 

 furnish important systematic characters. The urediniospores are easily 

 separated from their stipes and are disseminated by winds, as are the 

 aeciospores. Immediately after their maturity they are capable of 

 germination, but lose this ability after a few months. True overwinter- 

 ing of urediniospores is known with certainty in only a few cases, as 

 in Uredinopsis Struthiopteridis and Kuehneola albida. In continental 

 climates, they may have the opposite task of serving as resting spores 

 during a drought. In all these cases, they possess a very strong brown 

 wall, a persistent stipe and are called amphispores. 



Germination may take place within very wide limits of temperature, 

 but the ability to infect lies within narrow limits, i.e., in the neighborhood 

 of the optimum for germination. At germination one or more germ 

 tubes penetrate the stomata of the new host and there again develop a 



