SECT. III. THE PITH. 323 



tube, as in the stem of common Hemlock. In the 

 young shoot or plant it is green, changing to a bright 

 white with age ; but in the mature parts of the 

 Walnut-tree it is brown, in the Berberry it is 

 yellow, and in the root of Calamus aromaticus it is 

 red. 



The structure of the pith is precisely similar to Similar in 



, , ~ | , structure 



that of the pulp, being composed or an assemblage to the 

 of hexagonal cells containing a watery and colour- pup> 

 less juice, or of cellular tissue and a parenchyma. 

 The pith of the Elder is well calculated for the pur- 

 pose of microscopic inspection, as existing in great 

 abundance, and exhibiting the figure of the cells on 

 a large scale ; though it does not always follow that 

 plants which have the most pith have the largest 

 cells. But the cells of the centre are larger than 

 those of the circumference, where they are generally 

 somewhat compressed, as may be seen by placing 

 under the microscope a thin slice taken from the 

 horizontal section of the pith of almost any plant. 

 But it is only in the young and tender shoots or 

 plants that the cells are filled with a fluid ; for in 

 the aged trunks and branches the parenchyma has 

 escaped, and the cells are left empty. 



But if the structure of the pith is so precisely Why de- 

 similar to that of the pulp, why, it may be asked, is S^JjJ. 

 it to be designated by a different name ? Perhaps P er name - 

 the central and insular situation which the pith 

 always occupies is a sufficient reason. But the 

 texture of the membrane composing the cells of the 



Y 2 



