32 Disunion of contiguous Layers in the 



II. — On the Disunio?i of contiguous Layers in the Wood of Exoge- 

 nous Trees. By Rev. J. S. Henslow, M. A., Professor of Botany 

 in the University of Cambridge. 



In his Introduction to Botany, (second edition, p. 267,) Br Lind- 

 ley concludes his account of the different theories which have been 

 proposed to explain the manner in which the concentric layers of 

 Exogenous trees are formed, by the following remark : " As this is 

 one of the most curious points remaining to be settled among bota- 

 nists, and as it is still as much open to discussion as ever, I have 

 dwelt upon it at an unusual length, in the hope that some one may 

 have leisure to prosecute the inquiry." It is very plain that Dr 

 Lindley himself has embraced the theory of Du Petit Thouars, to 

 which also several other botanists of distinguished reputation are 

 equally inclined to assent. 



This theory I need not dwell upon, as every botanist interested 

 in physiological speculations is acquainted with it, and as it is fully 

 explained in the work to which I have just referred. But as this 

 theory cannot be considered so thoroughly established by any direct 

 experimental proof, which has placed it beyond all cavil, I propose 

 to examine one class of phenomena which have been considered 

 easily explicable by this theory, but not so readily to be explained 

 by any other which has been hitherto advanced. I allude to the 

 appearances occasionally met with, where the old trunk of a tree is 

 completely invested by the layers of wood composing an outer case 

 or new trunk of the same. In the New Series of the Annales des 

 Sciences, for 1835, (vol. iv. p. 126,) M. Meyen notices an obser- 

 vation of M. Wiegmann's to this effect : " L'observation d'un tronc 

 de Saule mort au milieu d'un tronc vivant est tres interessante ; ce 

 phenomene a ete observe par Lindley sur un Peuplier et s'explique 

 bien par la theorie de Du Petit Thouars." This allusion to Dr 

 Lindley refers to a paper of his in the Journal of the Royal Insti- 

 tution for May 1831, where he gives a description of a poplar ap- 

 parently presenting the " singular anomaly of one tree growing 

 within another." 



My own attention was more particularly directed to this account 

 about three or four years ago, upon meeting with a parallel instance; 

 and since then I have been on the watch for other examples which 

 might tend to throw further light upon the subject. The result of 

 my inquiries has satisfied me that this phenomenon does not afford 

 any direct evidence in support of the theory of Du Petit Thouars, 



