on the Formation of Organic Structure, 187 



points of great interest in the science of vegetable physio- 

 logy, to which I had particularly adverted in an article on 

 the sap-vessels (Vol. VIII. p. 142.). 



As Mr. Main claims my earliest notice, and I very much 

 wish to meet his views, I regret that I did not see his com- 

 munication until the 18th inst. (Dec. 1832). However, I 

 hope I may still be in time for your next publication. If I do 

 not greatly mistake, Mr. Main is the gentleman who penned 

 the review of the Domestic Gardener^ s Manual, inVol.VlI. p. 57. 

 (as I perceive by the index ; for I have not the number by me). 

 The article now before me is written in a style that identifies 

 it, to the best of my recollection, with that of the review ; and 

 I beg to return my best and cordial thanks for the kindly 

 spirit which breathes through both one and the other. 



The first question proposed by Mr. Main is grounded upon 

 an extract from a letter that I had received from Mr. Knight. 

 The hypothesis therein noticed belongs exclusively to that 

 gentleman. It is a deduction from his own positive observa- 

 tions and experiments : the very expressions quoted are, ver- 

 batim, those of Mr. Knight. " The nutriment absorbed 

 becomes the true sap or living blood of the plant, by exposure 

 to light in the leaf; it descends by the bark, by which the 

 matter that forms the layer of alburnum is deposited." 



I cordially, it is true, acquiesce in this opinion of the origin 

 of the alburnous deposit ; still, however, although Mr. Main 

 disclaims as inadmissible the answers of any other person but 

 myself to his questions, he surely cannot, upon due consider- 

 ation, refuse to receive Mr. Knight's own testimony in support 

 of a hypothesis originally his own. I, therefore, must take 

 the liberty to state the following question, and respectfully to 

 request Mr. Main to afford it his most serious reflection ; and 

 not to stop there, but to make the fact of which it treats the 

 subject of close investigation and critical experiment. This 

 will not be difficult during the ensuing spring ; and the results 

 may be fully ascertained in the course of a few months. 



I ask, then, has not Mr. Knight asserted, in the Philoso- 

 phical Tra7isactio7is, that, having partially detached strips of 

 bark of the walnut tree, of several inches' length, from the 

 alburnum, in the spring, he introduced beneath such bark 

 two folds of paper, each of which was coated on both sides 

 with bees' wax ; so that such strips of bark were placed 

 wholly out of contact with the alburnum, or other bark of 

 the tree, except at their upper ends. Air and light were 

 excluded by a covering of clay till autumn, when as much 

 alburnum was deposited upon the paper, along the whole 



