Rev. P. Keith on the Pith of Plants. 77 



Ylll.—Ofthe Pith of Plants, By the Rev. Patrick 

 Keith, F.L.S. 



The pith, as every body knows, is that soft and spongy sub- 

 stance which occupies the centre of the vegetable column, in 

 which it is inclosed as in a sheath. In some plants it is close 

 and compact, as in the willow ; in others it is loose and inter- 

 rupted, as in the walnut ; in some its diameter is large in pro- 

 portion to that of the trunk, as in the fig and elder ; in others 

 it is very small, as in the oak and elm. Much has been said 

 concerning its functions, and many opinions have been ha- 

 zarded. 



But the only points of view in which I mean to regard it 

 at present are, first, the extent to which it may be said to oc- 

 cupy the centre of the plant ; and secondly, that of its dimi- 

 nution or obliteration in aged subjects. 



1st. Does the pith occupy the root or any part of the root, 

 as well as the stem and branches ? Before we proceed further, 

 it will be well to circumscribe the limits of the terms root and 

 stem. — '^ That part of the axis which forces its way down- 

 wards, constantly avoiding light, and withdrawing from the 

 influence of the air, is the descending axis or root ; and that 

 which seeks the light, always striving to expose itself to the 

 air, and expanding itself to the utmost extent of its nature to 

 the solar rays, is the ascending axis or stem.^' Such is the de- 

 finition of Dr. Lindley (Introd. 45.) with which we rest con- 

 tent. 



Now though it seems to have been the opinion of the ear- 

 lier botanists that a pith is present in the root of all plants 

 as well as in the stem ; yet M. Dutrochet, (^ Recher. Anat.' 

 p. 13.) with some other modern botanists, deny its existence in 

 the root of exogens altogether. 



With a view to satisfy myself on this point I began in 1836 

 to look out for subjects of inspection. I cannot say that I 

 met with anything that could be fairly called a pith in the roots 

 of full-grown or aged trees. I thought I had perceived a pith 

 in the root of a plant of Berberis communis, which I inspected 

 many years before the period above stated, with a different 

 object in view ; but as it was merely a matter of recollection, I 



