ON THE FORMATION OF THE BARK OF TREES. 137 



annually generated, derives its origin from the older bark ; and the 

 interior part of this new substance is annually transmuted into alburnum, 

 or sap wood ; whilst the exterior part, becoming dry and lifeless, forms 

 the exterior covering, or cortex. 



The opinions of Grew do not appear to differ much from those of 

 Malpighi ; but he conceives the interior bark to consist of two distinct 

 substances, one of which becomes alburnum, whilst the other remains in 

 the state of bark : he, however, supposes the insertments in the wood, 

 the " utriculi" of Malpighi, and the " tissu cellulaire" of Duhamel, to 

 have originally existed in the bark. 



Hales on the contrary contends, that the bark derives its existence 

 from the alburnum, and that it does not undergo any subsequent trans- 

 formation. 



The discoveries of Duhamel have thrown much light on the subject ; 

 but his experiments do not afford any conclusive 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 subsequently changes into alburnum, 

 though riot 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. 



Duhamel has shown, 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 Wych elm (Ulmus montana) in shady situations, when no covering what- 

 ever was applied. A glareous fluid, as Duhamel 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 matter, which enters into the composition of this cellular sub- 

 stance, is evidently derived from the alburnum. 



* Traite d' Anatomic et de Physiologic vegetales. 



