Mr. Knight on the Formation 
104 
on the subject ; but his experiments do not afford any conclu- 
sive result, and some of them may be adduced in support of 
either of the preceding hypotheses : and a modern writer 
(Mirbel*) has endeavoured to combine and reconcile, in some 
degree, the apparently discordant theories of Malpighi and 
Hales. He contends with Hales, that the alburnum gives 
existence to the new layer of bark ; but that this bark subse- 
quently changes into alburnum, though not precisely in the 
manner described by Malpighi. 
So much difference of opinion, amongst men so capable of 
observing, sufficiently evinces the difficulty of the subject they 
endeavoured to investigate : and in a course of experiments, 
which has occupied more than twenty years, I have scarcely 
felt myself prepared, till the present time, even to give an 
opinion respecting the manner, in which the cortical substance 
is generated in the ordinary course of its growth ; or repro- 
duced, when that, which previously existed, has been taken 
off. 
Du Hamel has shewn, that the bark of some species of 
trees is readily reproduced, when the decorticated surface of 
the alburnum is secluded from the air ; and I have repeated 
similar experiments on the apple, the sycamore, and other 
trees, with the same result ; I have also often observed a 
similar reproduction of bark on the surface of the alburnum 
of the fVych elm (Ulmus montana) in shady situations, when 
no covering whatever was applied. A glareous fluid, as Du 
Hamel has stated, exudes from the surface of the alburnum : 
this fluid appears to change into a pulpous unorganised mass, 
which subsequently becomes organised and cellular ; and the 
* Traits d’Anatomie et de Physiologie vegetales 
