258 



to the most simple for digesting vegetable food, so those of the bul- 

 lock, camel, and whale, are links from the ruminating to the most 

 simple stomachs for digesting animal food ; and the camel's stomach 

 is the most important link in each series, the contraction peculiar to 

 its fourth cavity making it intermediate between the bullock and the 

 whale. 



Although the above facts appear to throw some light on the di- 

 gestion of different kinds of food, they also present difficulties which 

 must remain to be explained when further progress has been made 

 in the investigation. It is in general admitted, that animal substances 

 do not require so long a process to convert them into chyle as vege- 

 tables ; and hence the stomachs of carnivorous animals are in general 

 most simple : but why the whale tribe, which live on fish that are 

 very readily converted into chyle, should have a more complex sto- 

 mach, it is not easy to explain. What further uses, in regard to 

 other secretions, these preparatory stomachs may have, are foreign 

 to the design of the present paper, which Mr. Home considers as a 

 continuation of a series of observations on digestion, and hopes to 

 extend further at some future opportunity. 



On theFormation of the Bark of Trees. In a Letter from Thomas Andrew 

 Knight, Esq. F.R.S. to the Right Hon. Sir Joseph Banks, K.B. 

 P.R.S. %c. Read February 19, 1807. [Phil. Trans. 1807, p. 103.] 



An extraordinary diversity of opinion having prevailed amongst 

 naturalists most capable of correct observation, respecting the pro- 

 duction and subsequent state of the bark of trees, Mr. Knight has 

 undertaken to investigate the subject : but such are the difficulties 

 of the subject, that, in a course of experiment which has occupied 

 more than twenty years, he has scarcely felt himself prepared, till 

 the present time, even to give an opinion of the manner in which 

 the cortical substance is either generated in the ordinary course of 

 its growth, or re-produced when that which previously existed has 

 been taken off. 



Du Hamel had shown, that the bark of some species of trees is 

 readily re-produced when the decorticated alburnum is secluded 

 from the air. Mr. Knight has repeated these experiments on the 

 apple, the sycamore, and some other trees, with the same result ; 

 and has also observed, that the wych-elm, in moist and shady situa- 

 tions, will frequently re-produce its bark when no covering whatever 

 has been applied. 



A glairy fluid (as Du Hamel justly observes) exudes from the sur- 

 face of the alburnum, which appears to change into a pulpous or- 

 ganized mass, and subsequently becomes organized and cellular, 

 facts which are extremely favourable to the opinion of Hales, that 

 the bark is derived from the substance of the alburnum. But other 

 facts may be adduced which lead to a contrary conclusion ; since 

 the internal surface of pieces of bark, when detached from contact 

 with the alburnum, but remaining united to the tree at their upper 



