LARCH APHIS. 225 



great evil are four : — fungus ; frost-bite ; " Blight," or Chermes- 

 attack ; and deranged circulation of the sap ; and (without 

 venturing to offer a definite opinion) many circumstances 

 appear to point to this last, which may arise from any cause 

 affecting the health of the tree, but especially from weather 

 influences, such as rapid alternations of heat and cold, 

 moisture in the air, and want of sufficiency of sunlight, as at 

 least very possibly the cause of the blister. 



Prof. De Candolle observes that, as an Alpine tree, the 

 Larch is singularly free from disease, and the trunks remark- 

 ably healthy ; and that, though sometimes Larches may be 

 seen having a wound of " resinous cancer," it seems to proceed 

 from some accidental cause, such as a blow when the tree was 

 in full sap ; and after noting that he considers the cause of 

 diseases in British Larches must originate from some differ- 

 ence in the physical structure or culture of British or Alpine- 

 grown trees, he observes: — "The want of a sufficiently 

 intense light, owing to the obliquity of the solar rays, and to 

 the opacity of the atmosphere ; and the over-damp state of 

 the latter, appear to me permanent causes, which, in your 

 climate, must predispose the Larch to a kind of watery 

 plethora." — (See Loudon's 'Arboretum,' vol. iv., p. 2384.) 



It has also been observed that a form of bhster affects 

 young trees after being transplanted, in which case also the 

 regular circulation of the sap is disturbed. 



By examination of diseased specimens (although this is far 

 from affording a complete view of all forms of the disease), 

 the formation of the bhster may be traced backivards, from 

 the large open diseased wound, to the swelling just bursting, 

 and not yet burst ; and then (keeping as a guide the similarity 

 of the diseased spots as shown microscopically) from a patch 

 beneath the bark with no external swelling, to a few small 

 spots connected by a canal, or to a single spot filled with 

 brown disorganised tissue, which appears to me to be the 

 origin of the evil. 



In these spots (as far as observable in all the specimens 

 examined by means of a quarter-inch object glass) there was 

 no trace of Mycehum, or of any kind of fungoid presence ; 

 after a time, when the blister has become an open wound, it 

 is impossible to say that Corticium amorplmm (from which 

 "blister" has been conjectured to arise), or other fungoid 

 growths, may not be present, just as Peziza or other fungi may 

 be found on the bark ; but in all the first states of the blister 

 that I have examined, whilst still it had not burst into an open 

 wound, there has not been any such presence. 



In these cases— that is, where the " blister" was originating 

 from a mere speck — it differed markedly from the effect of 



Q 



