36 On the Formation of the Bark of Trees. 



thev endeavoured to investigate : and in the course of experi- 

 ments 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 sub- 

 stance is generated in the ordinary course of its growth ; or 

 reproduced, when that, which previously existed, has been 

 taken off. 



Du Hamel 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 chn {Ulnms viontnna) in shadi/ silnatlonsy 

 when no covering whatever was applied. A glareous fluid, 

 as Du Hamel has stated, exudes from the surface of the al- 

 burnum : this fluid appears to change into a pulpous unor- 

 ganised mass, which subsequently becomes organised and 

 cellular; and the matter, which enters into the composition 

 of this cellular substance, is evidently derived from the al- 

 burnum. 



These facts are therefore extremely favourable to the 

 theory of Males; but other facts may be adduced which 

 are scarcely consistent with that theory. 



The internal surface of pieces of baik, when detached from 

 contact with the alburnum, provided thev rcniaiii united to the 

 tree at their upper ends, much nuire readily generate a new 

 bark, than the alburnum does under similar circumstances : 

 a similar fluid exudes from the surfaces of both, and the 

 same phenomena arc observable in both cases. The cellular 

 substance, however, which is thus generated, though it pre- 

 sents every external appearance of a perfect bark, is inter- 

 nally very impLvfectly onianiscd ; and the vessels which con- 

 tain the true sap in the bark, are stiil wanting ; and I have 

 found, that these may be made, by appropriate management, 

 to traverse the new cellular substance in almost any direction. 

 When I cut off" all communication above, and on one side, 

 between the old bark and that substance, I observed, that 

 the vessels proceeded across it, from the old bark on the 



other 



