672 Microscopical Essays. 



pith, arc all one piece of work, rilled up in divers manners with 

 ,the veffels. 



The bark and the wood grow thicker every year, while the 

 pith, on the contrary, grows more {lender, fo that- in a branch of 

 one year it is of a larger fize than it is in the fame branch when 

 two years old, and fo on. In very young branches, while in an 

 •herbaceous (late, the pith forms the greateft part of it's fubflance ; 

 £mt when the fibres are ftronger, the pith becomes lefs fucculent, 

 and furrounded with a tube of wood; when the branch is arrived 

 to a certain age. it is fo compreffed as to be almofl annihilated. 

 In examining different branches that proceed from others in their 

 firft ftate, a fmall communication between the pith of the one 

 and the other will be found ; but this communication is generally 

 entirely clofed up in the fecond or third year * The cells of 

 which the pith is formed are at firft entirely one conneaed body; 

 but as the plant grows up, it is often fo broke and ruptured, as to 

 remain no longer a continuous Jubilance. 



This, as well as many other particulars in the hiftory of the 

 pith, corroborates the opinion of Dr. Hill,+ who thinks it is 

 formed for the purpofe of moiflening the clufters of the corona, 

 and regulating its extenfion ; it has been fuppofed coeval with, 

 or primordial to all the other parts, but he thinks it is poftnate, 

 and comes after them in the order of time, as well as in it's ufes. 

 That exhaled air gives origin to it's blebs, and the thicknefs of the 



juices 



* Duhamcl Phyfique des Arhies, torn, l, p. 38. 

 + Hill's Con-ftru&iQn of Timber, p. 66. " 



