5J42 FORMATION OF THE BARK OF TREES. 



num, whilst the other remains in the state of bark: he, how- 

 ever, supposes the insertments in the wood, the " utriculi" of 

 Malpighi, and the " tissu cellulaire" of du Hamel, to have 

 originally existed in the bark. 



Hales on the contrary contends, that the bark derives its 

 1 existence from the alburnum, and that it does not undergo 

 any subsequent transformation. 



The discoveries of du Hamel have thrown much light 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 

 subsequently 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 geuerated in the ordinary course of its growth; or repro^ 

 duced, when that, which previously existed, has been taken 

 off. 

 Bark of some Du Hamel has shown, that the bark of some species of 

 treesrepro- trees is readily reproduced, when the decorticated surface of 

 the alburnum is secluded from the air; and 1 have repeated 

 similar experiments on the apple, the sycamore, and other 

 trees, with the same result; I have also often observed a si- 

 milar reproduction of bark on the surface of the alburnum 

 of the wj/ch elm (ulmus montana) in shady situations, when 

 apparently no covering whatever was applied. A glareous fluid, as du 



from the al- Hamel has stated, exudes from the surface of the alburnum : 

 burnura. , . _ . , , , , 



this fluid appears to change into a pulpous unorganized mass, 



which subsequently becomes organized and cellular; and the 



* Traite d'Anatonrie et de Physiologic vegetale. 



matter, 



