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living tissue, and at 10 feet disappeared altogether, after which the 
remaining portion of the stem was healthy, having pith and wood layers 
regularly formed. Such has the remarkable permeability of vegetable 
tissue been proved to be, that every organ of a large tree has been killed 
over a space of nearly eight inches in the centre of its stem without kill- 
ing the plant, or impeding the circulation of sap to any great extent ; and 
although the wood portion so destroyed continued dead through a space 
of ten feet, it was again renewed to living tissue through the remaining 
portion of the stem—every organ in regular order. 
No. 2 diagram corresponds with specimens on the table, which were 
operated on for the purpose of showing how wood is formed when a grow- 
ing branch is isolated on the stem of a tree. A similar experiment had 
been commenced by Mr. N. Niven before he 
left the Garden, but sufficient time had not 
elapsed for its completion. 
The present specimens have been under 
operation during the last seven years, and 
prove more incontestably what has long been 
held as an axiom in vegetable physiology, 
namely, that the principal formation of new 
wood in dicotyledonous plants takes place 
in a downward direction, from the apex to 
the base of the stem. It was in consequence 
of Dr. Schleiden, of Jena, impugning this 
theory, so lately as within the last ten years, 
that I was desirous to prove or disprove it. 
In my formerpaper I was inclined to adopt 
his [views on this matter, to some extent, 
which further experience has shown me are 
erroneous. But his investigations on the 
origin of vegetable tissues, and his excellent 
definition of the true distinctions between 
the formation of the stems of monocoty- 
ledonous and dicotyledonous plants, are such 
as to claim for any theory he holds on this 
subject profound respect. Both he and Dr. 
Mohl have, through recent investigations, clearly proved that the vascular 
bundles of tissue which, combined, form so large a portion of the solid 
wood, grow in an upward direction, and enter the leaves from below up- 
wards, which is the converse of the theory so long held by Du Petit Thou- 
ars and his followers, that the vascular bundles were prolongations from 
the bases of the leaves downwards to the roots; hence they accounted for 
the increase of wood taking place in a vertical direction. But all my spe- 
cimens show that the new formation takes place in a horizontal plane 
from the axis, as well as it does in a vertical, thus proving that it 1s the 
alteration of tissue in the cambium layer towards the periphery of the 
stem which causes the increase in girth, and not so much owing to the 
growth of tissue downwards. 
At this point, I am led to refer to an article on sap circulation in 
plants by M. Trecul, published in ‘‘Comptes Rendus,” September, 
