Microscopical Essays. 675 



afierts, that if they are examined in winter, they often exhibit a 

 vermicular motion, which aitoni flies the fpeclator. 



Thofe who fuppofe the corona to contain the whole flruclure 

 of the tree in miniature, and that it is the embryo of future moots, 

 fuppofe it to contain the veffels proper for each part, a fubje6l 

 that mull be left to the decifion of future obfervers. 



Of the Vasa Propria Intima. 



Thefe are the only veffels which remain to be fpoken of. 

 They are large, confpicuous, and important ; their natural place 

 is in the blea, though they are fometimes repeated in the wood 

 and the corona. Their coats are thicker than thofe of any other 

 veffels. * It is not difficult, after a fuccefsful maceration, to fepa- 

 rate fome of thefe veffels from the blea ; in this flate they appear 

 perfect, cylinders, with thick white coats, of a firm, folid 3 and 

 uniform texture. 



It has generally been fuppofed, that each of thofe concentric 

 circles, which are to be obferved in the tranfverfe feclion of 

 almoft every tree, was the product of one year, or the quantity 

 of wood added to the tree in that fpace ; here, however, Dr. 

 Hill differs again from the general opinion. 



From what has been faid we may deduce the following general 

 ideas relative to the organization of trees. The moft obvious 

 and remarkable parts of a plant, or tree, are the root, the ffem, 



4 N 2 the 



Hill's Conftru&ion of Timber, p. 83 and 85. 



