148 VEGETABLE ORGANOGRAPHY. 



enable us to derive from it any important conclusions; 

 but it well deserves further attention. 



But in order to form a just idea of the pith, it is less 

 important to study the variations which it presents in 

 different plants, than to follow its whole history in a 

 single individual ; this is what we shall try to do in a 

 rapid manner. 



The pith of a very young shoot is a regular cellular 

 tissue, continuous, or strictly contiguous in all its parts 

 (one entire piece ; Grew, p. 120), and full of juice, which 

 renders it soft, and gives it a green herbaceous colour. 

 As vegetation proceeds, the cellules of this tissue become 

 empty, and dry up more or less quickly, according to the 

 species, and take a white, or, in some trees, a brownish 

 tint, and then, in different stems, one of the three fol- 

 lowing phenomena takes place : if the pith is tolerably 

 firm, and as its cellules may be small, or at least capable 

 of being stretched without tearing, as, for example, in 

 the Elder and Chestnut, it then dries up gradually, and, 

 at the end of the first year, takes the appearance of a 

 dried cellular tissue, but preserves all its original form. 

 In some trees, as the Oak, the cellular tissue of the pith 

 solidifies, and becomes hard and compact, but without 

 losing its primitive form. If the pith has large cellules, 

 or a tissue which is not capable of extension, it is then 

 broken transversely, or longitudinally, according as it is 

 ' drawn by the elongation or enlargement of the branch. 

 Thus in certain stems, such as those of the Walnut, the 

 / common Jessamine, &c., the elongation of the young 

 shoots breaks the pith transversely, and forms, at the 

 end of the first year, little transverse disks of dried pith, 

 separated by so many disciform cavities. 



If, on the contrary, the increase in diameter is propor- 

 tionally greater than the elongation, the pith then splits 

 longitudinally, as in the Thistle, Phlomis, and, in general, 



