186 PITH MAY CONTKACT. [BOOK i. 



whether they are lateral or terminal, it is in direct or indirect 

 communication. Its tissue, when cut through, almost always 

 exhibits an hexagonal character, and is frequently larger 

 than in any other part. When newly formed, it is green, 

 and filled with fluid ; but its colour gradually disappears 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, indeed, 

 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 wholly 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 germi- 

 nation, 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 pro- 

 portion to the general rapidity of development of the vege- 

 table system : the pith, therefore, in the first-formed shoots, 

 in which it is so small compared with that in the branches 

 of subsequent years, may not be small because of the pressure 

 of surrounding parts; it may never have been any larger. 

 It is, however, probable that by degrees the calibre of the 

 pith is lessened by the contraction of the fibrous column 

 of wood that incloses it. This would seem to be so from the 

 following statement by the Rev. P. Keith : 



" On the 1st of June, 1836, I separated from the stool of 

 an ash-stock a stem of three years' growth. It measured 

 about nine feet in height, the growth of each year being dis- 

 tinctly marked, and measuring each about three feet in 

 length. The upper shoot, that is, the shoot of 1835, had a 

 diameter of fths of an inch, with a pith of ith at the widest. 

 The middle shoot, that is, the shoot of 1834, had a diameter 

 of T^ ths of an inch, with a pith of ^th ; and the lower shoot, 

 that is, the shoot of 1833, had a diameter of -^ths of an inch, 



