126 GROWTH. 



coloured, and are filled inside with a yellowish brown 

 matter, and the walls of the cells of the spiral vessels 

 are thick throughout, and the interior of them filled 

 with a brownish colouring matter," so that, as has been 

 remarked, " anatomical investigations show that in co- 

 loured wood the fibre itself shows also a like colour, and 

 the tint of the wood is not alone to be ascribed to the 

 medullary rays." (Meyen.) 



190. The existence of the embryo-buds already spoken 

 of, in which concentric layers of woody matter are seen, 

 and which are generated in the bark, militate strongly 

 against the theory of Du Petit Thouars, and this coupled 

 with a peculiar development of vascular structure, ac- 

 cording to M. Decaisne, to be seen in the Beet-root, 

 are considered by Dr. Lindley to be the greatest ob- 

 jections to the views under consideration. 



191. Returning to the former part of this subject, 

 we may now mention the objection to the theory that 

 new wood is formed from out of cambium. " In many 

 Exogens of tropical countries, wood is not deposited in 

 regular circles all round the axis, but only on one side 

 of the stem or along certain lines upon it ; were it a 

 deposit from the bark, or a metamorphosis of cambium, 

 it would necessarily be deposited with some kind of 

 uniformity. In Endogenous trees there is no cambium, 

 and yet wood is formed in abundance, and in the centre, 

 not in the circumference, so that bark can have in such 

 cases nothing to do with the creation of wood. 

 (Lindley.) 



192. In regard to these objections we can only allow 

 that the last-mentioned one that Endogenous plants have 

 no cambium, and yet have wood, militate against this 

 view of the question ; but even here if we allow, and 

 which we at present must do, a complete difference as 

 regards the disposition, arrangement, and relationship to 

 each other of the elementary structures in Exogenous 

 and Endogenous stems, may we not also believe that 

 such difference only exists as dependent upon a particu- 

 lar way in which these structures have their origin, and 



