XX 



CONTENTS. 



PAGE 



authors, e.g., M. Du Petit-Thouars, Mr Appleby, Dr Ware, 

 M. Mirbel., M. Gaudichaud, Professor Owen. 



LETTER XII 122 



Objections to the theory considered. - Greneral nature and 

 grounds of these. Nature of the woody layers annually 

 formed in Exogens — these layers serve various purposes — 

 still they are both virtually and actually roots. Mode in 

 which they are evolved peculiar. Admission that they are • 

 primarily formed in situ and from the cambium, as main- 

 tained by Mirbel. Relation of the cambium to the bud — 

 the cambium continuous with and an integral part of the bud 

 — an extension of it in adaptation to the circumstances and 

 objects of existence of exogenous tree-plants. Constitution 

 and structure of the cambium — product of its development ; 

 this not wholly evolved in situ— 2, certain portion growing 

 and creeping downwards. This latter portion undeniably 

 root and specially the rooi— the former, or that formed in • 

 situ, a provision for bringing the root proper into relation 

 with the shoot emanating from the bud. No natural or 

 structural difference between the part above ground and 

 the part under ground ; the tissue of the one continuous 

 with the tissue of the other, and identical throughout with 

 roots. Mode in which the cambium is developed into cel- 

 lular and woody tissue. 



LETTER XIII 134 



Evidence in favour of the assumption that the woody layers in 

 Exogens are of the nature of roots. Whenever circum- 

 stances admit of it, the woody tissue of the trunk grows 

 and comports itself as roots do ; processes of slipping and 

 layering. North Stoneham elm, with roots filling up a 

 chasm in the bole — other examples of this. Aspect of a 

 piece of hardwood timber denuded of its bark. Relation 

 of branches spreading southwards to roots spreading north- 

 wards. Disposition of the woody fibres in the thorn, the 

 poplar, the beech, and the yew. Spontaneous division of 

 trunk of thorn into lesser stems. Fibres descending from 

 a branch in Draccena and spreading over the trunk ; graft 



