168 Detail of Experiments 



develope leaves^ and form shoots^ as before : but it remains to be 

 proved how long they will continue to vegetate above the rings. 

 If I may give an opinion, I am disposed to think that, whilst to 

 one species of tree such operations would produce almost imme- 

 diate death, in others the result would be very different. Drawing 

 an analogy from animal life, as to the tenacity with which it is 

 held by different species, there is every reason to conclude that 

 the same will be found to hold good with respect to tena- 

 city of vegetable life in different species of plants also. We 

 have, for instancf , the willow, elm, and several other trees, that 

 will continue to produce leaves and shoots from the trunks for 

 one, two, and even three, years after they are cut down, and 

 laid along the ground. The descent of the sap, described 

 ni^g. 26. as descending from above, through the pith and in- 

 ternal layers of wood, even before the developement of leaves, 

 appears to me both curious and important ; especially when 

 I recollect that the received opinion, according to Mirbel and 

 Knight, is, that the sap descends " through the cortical vessels ; " 

 the latter admitting that, " when interrupted by the destruction 

 of a ring of bark, he supposes that part of it escapes downwards 

 through the alburnum ; but, before forming any positive opinion 

 on this curious result, it is my intention, first, to follow it up with 

 additional experiments. 



In my first series of experiments, in 1835, on watching 

 the developement of what has been called the " new cortical 

 layers," usually appearing in the form of protruding bark, from 

 the upper edge of the rings, I was much interested to find some- 

 thing of a similar developement beginning to appear from below 

 also, and exactly from the same part betxveen the bark and the 

 alburimm, as that from above. Recollecting no itistance of any 

 author having noticed this appearance, and remembering that 

 statements peifoctly opposite to such an appearance had been 

 made ; as, for instance, in Sir H. Davy's fourth lecture in his 

 Agricultural Chemistry, he says that, " when new bark is 

 formed, to supply the place of a ring that has been stripped off, 

 it first makes its appearance from the upper edge of the wound, 

 and spreads slowly downwards ; but no nevj matter appears from 

 below rising upwards." A similar opinion appears to have been 

 entertained by Du Hamel and others, who " cut a ring of bark 

 from a branch, and found that, by thus stopping the descent of 

 the pulp, the upper part extended and healed, while the lower 

 remained stationary." On a close examination of the two ap- 

 pearances alluded to, I thought I could distinguish a marked 

 difference between them ; the one appearing like small gems or 

 buds, the other like to the rudimental structure of roots. It 

 immediately struck me that here might be tiw perfoctly dis- 

 tinct external principles, proceeding in opposite directio7is through 



