STRUCTURE.] PITH IN ROOTS. 187 



with a pith of -roth. Now, as the shoots of the several years 

 were equally luxuriant, and the youngest a year old, the pith 

 ought, by hypothesis, to have been of the same dimensions in 

 all of them. Yet it was gradually smaller and smaller from 

 the youngest to the oldest, though it was undoubtedly of 

 equal diameter in the first year's growth of each. For the 

 shoot of a single year, from a different stock, gave a diameter 

 of pith equal to that of the upper shoot of the above stem ; 

 and poles of twelve years old gave still a diminishing dia- 

 meter when inspected towards the base." Whence he inferred 

 that the pith keeps shrinking, from one cause or other, 

 long after the period of the first year's growth. This may 

 be admitted without acquiescing in the statement formerly 

 made by Mirbel, that pith diminishes in diameter, "by 

 being converted first into longitudinal tubes and then into 

 wood/' an hypothesis which seems to rest on no satisfactory 

 observation. 



According to Dutrochet and others, the pith does not 

 extend into the root of Exogens; and it certainly occurs 

 there, when woody, in very minute quantity, if at all. Mr. 

 Keith, however, seems to have shown that its presence has 

 been sometimes overlooked, for he finds it in many seedlings. 

 Of this he gives the following examples : 



" I took up a seedling of the Beech Tree, Fagus sylvatica. 

 The seed leaves were still attached to it, and were fully 

 expanded ; and the stem on the horizontal section was divi- 

 sible into bark and bundles of woody fibre, together with a 

 central pith and spirals. All this is what was to be expected; 

 and the next thing remaining to be done was the inspection 

 of the roots of the said seedlings, which was now undertaken. 

 In the above specimens, this root measured from two to 

 three inches in length, with a good many lateral fibres, and 

 on a horizontal section exhibited like the stem, a bark, a 

 circular layer of woody fibre, but without spirals, and a 

 central or axial mass, which mass differed in nothing visible 

 from the central mass of the stem, whether as relative to its 

 colour or to its spongy and cellular texture. On this account 

 I have no hesitation in pronouncing it to be a true and legi- 

 timate pith, though lodged in the descending axis. If it be 



