370 Dr. Lankester's Remarks on Du Petit Tfiouars's 



are separated from each other for practical purposes, there 

 can be httle doubt as to their common origin. 



But whilst botanists have admitted that the tissues are thus 

 developed from a common form, there is still considerable dif- 

 ference among them as to the precise manner in which this is 

 accomplished. This remark applies especially to the forma- 

 tion of the woody tissue, and the varied opinions of botanists 

 on this subject have given rise to the interminable discussions 

 on the origin of wood. Although much has been written on 

 this subject, the whole question may be summed up in the 

 words of DeCandolle, " Either there descend from the top of 

 a tree the rudiments of fibres which are nourished and deve- 

 loped by the juices springing laterally from the body of wood 

 and bark ; or new layers are developed by preexisting layers 

 which are nourished by the descending juices formed in the 

 leaves." The latter is the view adopted by DeCandolle, whilst 

 the former originated with Du Petit Thouars, and these views 

 are respectively advocated by Professors Henslow and Lindley 

 in their works in this country. 



In order to give a clear view of opinions on this subject I 

 extract the following passages from Dr. Lindley's work. After 

 referring to some experiments of Knight, he says, " An in- 

 ference is drawn that the wood is not formed out of tlie bark 

 as a mere deposit from it, but that it is produced from matter 

 elaborated in the leaves and sent downwards, either through 

 the vessels of the inner bark along with the matter for form- 

 ing the liber by which it is subsequently parted with ; or that 

 it and the liber are transmitted distinct from one another, the 

 one adhering to the alburnum, the other to the bark. I know 

 of no proof of the former supposition ; of the latter there is 

 every reason to believe the truth." 



In giving the views of Du Petit Thouars, he uses the follow- 

 ing passage : " It is not merely in the property of increasing 

 the species that buds agree with seeds, but that they emit roots 

 in like manner ; and that the wood and liber are both formed 

 by the downward descent of bud-roots, at first nourished by 

 the moisture of the cambium, and finally imbedded in the 

 cellular tissue, which is the result of the organization of that 

 secretion". From these passages I think we must infer that 



