252 HOST PENETRATION 



changes in aggressiveness or pathogenicity which they may un- 

 dergo as the'result of growth on wounded tissues. This matter 

 has'been the subject of experimentation and speculation by many 

 students of fungi. Salmon ( 1905) made the observation that races 

 of Erysiphe graminis occurring on various genera of grasses are 

 morphologically indistinguishable yet cannot be made to infect 

 reciprocally when cross-inoculated from one genus to another. 

 If, however, he wounded the leaf by cutting away a small piece of 

 tissue or by applying a hot needle to its surface and then placed 

 the spores on the surface opposite the wound, ready infection 



resulted. 



Many have concerned themselves with what may be a closely 

 related problem in trving to account for the inability among het- 

 eroecious rusts of basidiospores to infect the telial host. The mere 

 oeneralization that aggressiveness is enhanced or rejuvenated by 

 sexuality does not appear to constitute a satisfactory explanation. 



HAUSTORIA AND THEIR SIGNIFICANCE 



Penetration of tissues by fungi is also concerned with host- 

 parasite relations after the pathogen has pierced the cuticle or 

 epidermis, the first line of defence. Some species remain entirely 

 intercellular; others are intercellular but possess intracellular haus- 

 toria; and in others the mycelium itself courses intracellularly from 

 cell to cell. Our immediate interest is in the haustorium-forming 

 species. This group includes such obligate parasites as the downy 

 mildews, powderv mildews, rusts, and smuts but is not confined 

 to obligate parasites, since haustoria have been observed in Coc- 

 ci >mvces, Diplocarpon, and other genera. Among the rusts, in- 

 tracellular mycelium has been observed in one species only, 

 namely, the short-cycled form of Gymnoconia interstitialis. In 

 this species, which is systemic, Pady (1935) described peculiar 

 intracellular elements which functioned to establish the fungus 

 in the host. They were therefore interpreted as being haustoria! 

 in nature. An elaborate account by Rice (1927), dealing espe- 

 cially with haustoria of rusts, contains much of value regarding 

 the structure and function of haustoria in general. 



Haustoria vary in form among the different species of fungi, 

 bein^ spherical in the simplest forms and variously branched and 

 tabulate in the most complex ones. Their size indicates conform- 



