32 Disunion >of contiguous Layers in the 
II. On the Disunion 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,) Dr 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 j 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, 
