3 94 Retrospective Criticism. 



Mr. Niven on the Growth of Dicotyledonous Trees, (p. 161.) — As one of 

 your old correspondents, I, on the present occasion, beg leave to thank you 

 for giving publicity to Mr. Niven's excellent paper on the growth of dicoty- 

 ledonous trees, read at a meeting of the British Association in Liverpool on 

 Tuesday, September 12, 1837. I have also to congratulate your readers 

 that that paper has been accompanied by expensive engravings of Mr. Niven's 

 experiments, which renders the whole so clear and explicit, that " he who 

 runs may read." 



Every one acquainted with the manner in which a wound on a tree is 

 healed, will at once agree with Mr. Niven, that his delineations of the pro- 

 cesses are true to nature ; and that the descending process is an attempt to 

 reach the ground and to form roots ; and also that the swelling, or lip, at the 

 bottom of the wound is a natural endeavour to rise, and resolve itself into 

 shoots. Both these circumstances are facts, and have been fully proved by 

 Mr. Niven's experiments, as appears from his figs. 21, 22, and 23. p. 164. 

 and 165. 



That such experiments have been made before, and attended with similar 

 results, is perfectly true; but it appears that erroneous explanations have been 

 given of them ; because the descending processes have usually been called ar- 

 rested accumulations of the elaborated sap, while not a word of explanation has 

 been given of the ascending process, although it is evidently (except so far as 

 is excepted by Mr. Niven) a body of an exactly similar nature in texture and 

 consistence. Yet, as no elaborated sap could descend to the lower lip, except 

 through the solid wood (which the favourers of the doctrine of the descent of 

 the sap say it never does), the swelling of the lower lip has remained 

 a physiological puzzle, merely because none of the theorists could beheve 

 that any vegetable membrane could be enlarged by the accession of crude 

 sap from the root ; an idea completely in the teeth of every fact observable 

 in the growth of vegetables. 



But we are neither beguiled nor staggered by Mr. Niven's representations. 

 He has plainly described what he witnessed, and has given proofs of the 

 conclusions he arrived at, on careful consideration of the results. He wisely 

 offers no theoretical opinion upon the invisible courses, or changes of the sap ; 

 nor does he attribute to the latter that inconceivable property of being 

 organisable ^^ey se. He admits that the cambium exists between the cortical 

 layers and alburnum in February. In May, he observes this same body 

 increasing gradually into bark and wood, from the top to the bottom. He 

 also sees a corresponding flow, but less rapidly, from below. He does not, 

 however, speak of it as a liquid gushing from the upper side, or springing 

 fi'om the lower one ; but as it actually is, namely, tender bark and wood, 

 which gradually thicken ; and, spreading, eventually cover the whole face of 

 the scar ; and which new bark and wood, he shows, are capable of emitting 

 either roots or shoots. 



If, at the commencement of his experiment fig. 20., he had deprived the 

 tree of the whole of the cortical layers, with every vestige of the cambium, as 

 he did in fig. 21., it is evident that no generation of bark and wood would 

 have appeared on the naked alburnum, as it did in the first experiment. The 

 new healing processes would have only appeared at the upper and lower 

 edges, as Mr. Niven has so correctly represented. From all which statements, 

 it may be fairly inferred, that the cambium is the only vital membrane of the 

 system, and, of course, is the origin of all new accretion, whether of bark and 

 wood only, or roots and shoots also. 



With respect to what Mr. Niven has observed of the movement of the sap 

 in the interior of his windowed tree, and which remains for future obsei-vation 

 and experiment, I doubt not but that he will find the fluidity of the sap, at 

 an early season, always more or less according to the temperature of the 

 interior. The central parts of a trunk, and particularly of a very large one, 

 are always much warmer than the exterior ; and there the sap will be most 

 fluid ; and, moreover, when the tubular structure is so separated, oozings may 



