6S 



ORGANOGRAniY. 



BOOK I, 



the centre and the circumference, which are connected by tliin 



vertical plates of the same nature 



as themselves. The centi-al part 



(a, fig. 32.) is the pith, that of the 



circumference (h) is the bark, and 



the connecting vertical plates (c) are| 



medullar^/ rays. 



The pith is a cylindrical or an- 

 gular column of cellular tissue, 

 arising at the neck of the stem 

 and terminating at the leaf-buds, 

 with all of which, whether they are lateral or terminal, it is in 

 direct communication. Its tissue, when cut through, almost 

 always exhibits an hexagonal character, and is frequently 

 larger than in any other part. Wlien newly formed, it is 

 green, and filled with fluid ; but its colour gradually disap- 

 pears as it dries up, and it finally becomes colourless. After 

 this it undergoes no further change, unless by the deposition 

 in it, in course of time, of some of the peculiar secretions of 

 the species to which it belongs. It has been contended, in- 

 deed, by some physiologists, that it is gradually pressed upon 

 by the surrounding part of the vascular system, until it is 

 either much reduced in diameter or v/holly disappears ; and in 

 proof of this assertion, the Elder has been referred to, in 

 which the pith is very large in the young shoots, and very 

 small in the old trunks. Those, however, who entertain this 

 opinion, seem not to consider that the diameter of the pith of 

 all trees is different in different shoots, according to the age of 

 those shoots ; — that in the first that arises after germination, 

 the pith is a mere thread, or at least of very small dimensions 



— that in the shoots of the succeeding year it becomes larger 



— and that its dimensions increase in proportion to the gene- 

 ral rapidity of developement of the vegetable system : the pith, 

 therefore, in the first-formed shoots, in which it is so small 

 compared with that in the branches of subsequent years, is 

 not small because of the pressure of surrounding parts; it 

 never was any larger. 



The pith is always, when first forming, a uniform compact 

 mass, coimected without interruption in any part ; but the 



