112 MASSACHUSETTS HORTICULTURAL SOCIETY. 



decaying parts of plants, and for a long period produces no 

 serious harm, but again it gains such a foothold at times on cer- 

 tain plants, that nothing short of the destruction of the host will 

 stop its course. For several years I have been interested in 

 watching the habits of this fungus, and many times have been 

 constrained to look upon it as a harmless accompaniment of other 

 disorders ; and usually, I think, this is the case. But the present 

 winter some potted roses came under my observation showing at 

 first a few leaves which had been damped off by the fungus. 

 Gradually the mycelium invaded the more permanent tissues of 

 the rose stems until all parts of the plants were so thoroughly 

 infested with the parasite, and so badly disorganized in places, 

 that it is doubtful whether they coiild ever recover. De Bary has 

 shown how the mycelium of the Sderotinia libertiana, after it has 

 been aroused to activity by growth on the dead parts, is enabled 

 successfully to attack the living ones. 



This tendency is of course in many cases largely governed by 

 environment and the physiological condition of the host, but the 

 fungus makes use of these conditions, and for the time is enabled 

 by them to set such a pace of parasitism, that plants are more 

 easily invaded which would otherwise escape injury. 



Adaptatiox to New Conditions. — Close to this tendency 

 to fluctuation in virulence is the tendency tOAvard adaptation to 

 new conditions. This can be easily demonstrated in the case of 

 some species when attempting to grow them in artificial media. 

 For several years I have been attempting at various times to ob- 

 tain a culture of the leaf-spot of the quince and pear. Several 

 times the fungus has been found to germinate in nutrient agar, 

 but in no case would it develop more than a fcAv short threads in 

 this medium. The spores, after having been transferred to ster- 

 ilized pear fruit in culture tubes or to sterilized bean stems, 

 showed no signs of growth until after about three weeks, when 

 a minute tuft of mycelium could be observed at the point where 

 the spores had been transplanted. In the course of two or three 

 weeks more numerous spores characteristic of this fungus were 

 present, and the mycelium was quite well developed. In making 

 new transplantings from these spores to fresh sterilized bean 

 stems, a new crop of spores and well-developed stromata were 

 present in a little more than two weeks, and on transplanting the 

 spores from this second crop to fresh culture tubes of the same 



