330 



THE FUNGI WHICH CAUSE PLANT DISEASE 



aecia are on pines (Peridermium), while the other stages are 

 on dicotyledons. In the Gymnosporangiums the pycnial and 

 secial stages are on Rosacese; the telial on Juniperus and its kin. 

 While a few general rules can be worked out concerning host 

 relations there are many exceptions and to know one stage of 

 a heteroecious rust generally gives little or no clue to what its 

 complementary host may be. 



The mycelium of the rusts is usually intercellular and local though 

 in a few instances it is extensive and even perennial in the host. It is 

 abundantly branched, closely septate, gives off haustoria and usually 

 bears numerous oil drops which lend a yellow or orange color. 



Irritation by the mycelium often induces marked hypertrophy 

 or even witches' brooms or other deformation of the host. Hy- 

 pertrophy is most common with the aecia but may result 



from the telia as 

 well, as is conspicu- 

 ously shown in the 

 genus Gymnospor- 

 angium. In some 

 instances the whole 

 habit of the host 

 plant is altered by 

 the presence of the 

 mycelium so as to 

 render it almost tm- 

 recognizable, e. g., 

 the secium of Uro- 



Fio. 243. — Cross-section, showing infection from spore mtrnoc T.ioi evn V,-, 

 of P. asparagi. After Smith. myces piSl On iliU- 



phorbia. 



The host cells are seldom killed by the mycelium, which ab- 

 stracts its food supply from the carbohydrates and other nutrients 

 of the cell sap without direct injury to the protoplasm, though 

 ultimately there is serious effect upon both growth of the host 

 and its seed production. 



Cytology."* ISO. 189. 313-315 Dangeard^i^ and Sappin-Trouffy "" 

 showed that the mycelial cells of the rusts are binucleate, a condi- 

 tion which begins just below the aecium. The origin and signifi- 

 cance of this condition is of much interest. 



