NECROTIC DISEASES. 243 



transpiration current, and drought and inanition 

 supervene. 



Dying-back is frequently also a direct effect of 

 early frosts, which kill the thin twigs before the 

 " wood is ripened," as gardeners say. 



Dying-back is also a frequent result of direct 

 frost action on thin watery shoots or " unripe 

 wood," and is apt to occur every year in certain 

 varieties of Roses, for instance, in particular 

 situations, such as "frost-beds," or aspects exposed 

 to cutting winds, and so forth. The necrosis 

 which results may affect all the tissues, or only 

 the cortex and cambium, and the frequent accom- 

 paniment of all kinds of saprophytic Ascoinycetes 

 and moulds or other fungi is in no way causal 

 to the phenomenon. 



Dying-back may also be caused by fungi, and not 

 necessarily parasites, for cases are often observed 

 where saprophytes only are to be found in the 

 necrotic tissues of the cortex, having made their 

 way in through minute cracks, lenticels, etc. 



A simple case is often seen in Chrysanthemums, 

 Roses, etc., chilled and wetted to danger point, but 

 not frozen, during the nights of autumn. The 

 lowered resistance of the chilled tissues enables 

 fungi like Botrytis cinerea to gain a hold, and the 

 peduncles die-back with all the symptoms of 

 Necrosis, the fungus gaining power more and more 

 as its myceliun spreads in the dead tissues. 



Many other cases are known where wound- 

 fungi, such as Nectria, Cucurbitaria, Phoina, etc., in 

 themselves incapable of true parasitism, gain a hold 



