Presidential Address. F. T. Brooks. 29 



tist will have the courage to eliminate many of these so-called 

 genera, or establish a more adequate classification. 



No branch of systematic botany should be divorced from the 

 study of living plants, and just as those who classify flowering 

 plants cannot but be assisted in their labours by following the 

 work of geneticists and by watching the behaxiour of the li\ing 

 plants under different en\ironmental conditions, so, I think, it 

 will be of advantage if systematic mycologists will actually grow 

 certain groups of fungi under artificial conditions and will keep 

 in intimate touch with plant pathologists. An instance of the 

 lack of co-operation between fungal systematy and plant patho- 

 logy is afforded by the separation of the species Siereum 

 rugosiusciilum from Stereum purpureum by Burt*. This dis- 

 tinction is supported by Rea in his "British Basidiom\'cetae." 

 Stereum purpureum is "the primary cause of silver-leaf disease 

 of fruit trees in this countr}', and has hitherto been held to 

 include the form rugosiusculum now^ separated by these authori- 

 ties. Since Burt published his monograph on the genus Stereum 

 I have examined many collections of this fungus from different 

 substrata, and as far as investigation has proceeded, the chief 

 distinction between the two species according to Burt, namely 

 the presence of hymenial cystidia in rugosiusculum and not in 

 purpureum, seems to be an inadequate criterion. In none of 

 the specimens have the cystidia been so abundant as figured 

 by Burt for rugosiusculum, but, apart from this, there has been 

 every possible intergrade between forms possessing many 

 hymenial cystidia and forms devoid of them. Indeed, within 

 the limits of a single sporophore there are great differences in 

 the distribution of these hymenial hairs. From the pathogenic 

 standpoint I believe 5. rugosiusculum and S. purpureum are 

 one, and with aU deference to Burt and to the author of ' ' The British 

 Basidiomvcetae," I think doubt must remain as to whether S. 

 rugosiusculum is a valid species until more adequate e\idence 

 is forthcoming as to its structural peculiarities. 5. purpureum 

 and S. rugosiusculum would probably never have been separated 

 if the systematist had co-operated with a plant pathologist. 



There are innumerable other relations between mycology and 

 plant pathology, but only the briefest reference can be made 

 to these. The day has passed when it was considered sufficient 

 in investigating a plant disease of parasitic origin to determine 

 the causative agent. Nowadays that is merely the beginning 

 of a new series of investigations to elucidate precisely the con- 

 ditions under which the pathogen can effect an entrance into 

 the host, the exact mode of penetration, and the relationships 



* Burt, E. A., The Thelephoraceae of North America, xii. Ann. Missouri 

 Bot. Gard. vii, p. 127 (1920). 



