478 Mr. Lindley on an 



divided into two parts, and distributed two contrary ways : the 

 outer part falleth off towards the skin, and at length becomes 

 the skin itself: the inmost portion of the bark is annually dis- 

 tributed, and added to the wood. 3 This opinion has met with 

 many supporters, and is, I believe, even at this day, not uni- 

 versally abandoned. Du Hamel, as is well known, and after 

 him Mirbel, instituted the following experiment, in order to 

 determine the truth of Grew's opinion. If, they reasoned, a 

 metal plate is introduced between the bark and the wood in the 

 early spring, before the growth of the new year has commenced, 

 it ought to be covered by wood after a certain period, provided 

 the opinion of Grew be well founded. A plate of silver was 

 introduced, with the result that was anticipated. When exa- 

 mined, after the lapse of two or three years, it was found im- 

 bedded in the wood. At the same time that this experiment 

 seemed to prove the accuracy of the opinion that wood was 

 deposited by the bark, it also served to disprove the theory that 

 the bark was produced by wood. 



Satisfactory as the result of this experiment may appear to 

 have been, physiologists have long been aware that it was 

 liable to several objections ; and hence has arisen the third 

 hypothesis to which I have adverted, that the bark and the 

 wood each reproduces itself; the substance out of which the 

 annual addition is formed being supposed to be the viscid 

 secretion found betweeen the bark and the wood, and known by 

 the name of c cambium.' 



This view is that which is taken by many physiologists of the 

 present day. But if we consider that the tissue of both the 

 wood and the bark consists not only of cellular matter lying in 

 all directions, but also of vessels and fibres running in lines 

 parallel with the axis of development, and turning from their 

 course, if any obstacle is opposed to them, just as a current of 

 water when interrupted by stones or other obstructions, it 

 seems difficult to reconcile such a state of organization with the 

 idea of an induration of a mucous homogeneous deposit. 



The fourth mode of understanding the origin of wood and 

 bark namely, that they are caused by the descent of matter 

 sent down by the leaf-buds, is generally attributed to Darwin, 

 but may be also traced to Hales, who justly enough observes 



