By Mr. John Robertson. 177 



suppose ; but, for the purpose of ascertaining the fact with 

 more precision, I last summer attached some young shoots 

 infected by it, to two distant branches of a Red Magdalen 

 Peach tree, that was perfectly free from the disorder, and 

 grew on an eastern aspect ; the weather was then dry and 

 warm, and the wind south-easterly. After some time, the Mil- 

 dew made its appearance, on those parts of the tree to which 

 the shoots were fastened, and continued to extend itself there, 

 for the remainder of the season, but on repeating the experi- 

 ment, with as much attention in every respect, as before, on 

 a tree growing on the western or opposite side of the wall, it 

 failed to produce any effect, in consequence, I apprehend, of 

 the weather having become moist and cool, accompanied by 

 a westerly wind, which prevented the ripening and dispersion 

 of the seed. 



This year, 1821, I again repeated the same experiment, on 

 the western aspect, but no appearance of infection shewed 

 itself until an interval of four or five weeks had elapsed ; I 

 then observed the Mildew on a shoot below that to which the 

 infected one was attached, and with which it came in contact. 

 These various results tend to prove how much the appearance 

 of this disorder depends on contingent circumstances. 



The following instance also affords me reason to suppose, 

 that the seed of the Mildew is not absorbed, and circulated 

 in the vessels of the plant, to be again developed at a future 

 season, but rather preserved, by adhesion to its external sur- 

 face until a favourable opportunity may present for its vege- 

 tation. Last autumn a lot of maiden Peach trees in my 

 possession, were severely injured in their tops by the mildew, 

 but as the plants were excellent in every other respect, I this 



