ME. B. THOMPSON — ON PEACII- AND NECTAEINE-TPEES. 149 



the roots, which corresponded with the part which ultimately 

 died, were like those in the trees at Chatsworth, studded with 

 knots covered with thick bark, but instead of producing lenticels 

 were rough with abortive adventitious buds, which soon decayed 

 and communicated the seeds of decomposition to the subjacent 

 tissues. The wood beneath the bark was full of little cavities, 

 precisely of the same nature as those beneath the buds in the case 

 just mentioned, as was demonstrable in the specimens exhibited at 

 South Kensington. It was stated, then, that there were no cavities 

 in the knots of the Chatsworth roots ; but though this was true 

 at first, a fresh examination of them a few days after when dry, 

 showed exactly similar cells. An examination with the microscope 

 did not show the presence in either case of fungous threads in the 

 knots ; but every microscopist knows how difficult it is in many 

 cases to determine accurately whether spawn is present or no. 



After the case was brought forward on the 1st of May, it was 

 remarked, by a gentleman present at the Meeting, that no cause 

 had been assigned for the malady — a reproach which may be made 

 in animal as well as vegetable pathology. In how very few cases 

 comparatively is it possible to assign a cause for the origination 

 of disease, even in those cases where their treatment is perfectly 

 understood. Diseases, of various kinds, are transmissible by con- 

 tagion, and when once established do not readily die out, but 

 sometimes remain unaltered for centuries. But in most instances 

 who can assign the original cause ? So is it with the maladies of 

 plants. "Where they spring evidently from parasitic fungi, the 

 cause is often clear enough, and the remedy occasionally distinct, 

 though more frequently hopeless ; but there are doubtless many 

 diseases to which vegetables are subject whose origin will ever 

 remain unknown. The chlorotic disease in cucumbers, for 

 example, which has been so prevalent for some years, has baffled 

 every inquiry into its cause or origin. 



In conclusion it may be well to call attention to a case of 

 canker in apple-trees, which is closely analogous to that in the 

 Isle-of- Wight p each-trees, though it is the trunk which is attacked 

 and not the roots. Certain disks appear on the bark, which swells 

 and cracks, and at length give rise to a multitude of adventitious 

 roots. The tender spongelets of these soon decay, and the mis- 

 chief spreads in every direction till the tree falls a sacrifice. This 

 is only one of various forms of canker, and certainty not the most 

 common one; but it is clear] y deserving of especial notice here, 

 from its peculiar characters. 



