No. 496] ASPECTS OF THE SPECIES QUESTION 247 



think that this is not a joke, but that it is really stated 

 in earnest; because, I read it in Science. [Laughter.] 

 But I fancy botanists have not gone quite that far. 



Xow, just another instance that arose during an ex- 

 tensive series of culture studies; that is the case of the 

 Helianthus rusts. They have been described according 

 to the host plants on which they grow. Each species 

 of Helianthus and its close relatives appear to bear 

 a distinct kind of rust, which acts in cultures as if it 

 were an independent species. Yet any and all of these 

 will grow on Helianthus annuus, a so-called bridging 

 host. Are these different species different biological or 

 physiological species, or simply forms or races? Pos- 

 sibly it would be well to refer them to some sub-category, 

 as Dr. Britton has suggested. 



I think when we come to study other parasitic forms 

 thoroughly we shall find this condition equally true, for 

 instance, in the genus Cuscuta among flowering plants. 

 Furthermore it is probably true in many cases of plants 

 which are not parasitic. We had an illustration this 

 morning in the cultures of Penicillium exhibited by Dr. 

 Thorn, where characters of taxonomic importance were 

 developed according to the medium on which the fungus 

 was grown. 



It seems highly probable that a physiological cause 

 for variation, which may be considered as specific under 

 many circumstances, could be traced throughout the 

 vegetable kingdom. For purposes of study I assume 

 that in most cases of physiological species, they may 

 develop in the course of time into true taxonomic species, 

 having distinct morphological characters, and that in 

 cases such as I have enumerated, we are dealing with 



in the presentation of the subject by the previous 

 speakers apply more particularly, I think, to forms well 

 differentiated morphologically. But due recognition 



