360 CONSERVATIVE ORGANS. [LECT. VII. 



distinguishable from the wood, except by the spi- 

 ral vessels ; which have not yet been discovered 

 in any layer of formed wood subsequent to the 

 first; for their apparent existence in stems and 

 branches of several years' growth, is owing to 

 the lignification of the medullary sheath. Grew 

 and Hedwig, however, have represented them as 

 existing in the wood ; but, although I have 

 searched for them in every species of wood, yet 

 I have not been able to detect them ; and as the 

 same result has followed the investigations of Du 

 Hamel, Mirbel, Knight, Mr. Keith, and others, I 

 am inclined to regard my conclusion as correct. 

 The cells which are between the layer of spiral 

 vessels and the pith (Plate 6, fig. 7, d. e. 8.) ; and 

 which are the site of the colouring matter, when 

 this part of the stem is green, as it is in the 

 example now before us, have a cribriform struc- 

 ture. 



The spiral vessels of the medullary sheath vary 

 in their arrangement, and thus the widely separated 

 bundles they form in the Elder, give the canal of 

 the pith a furrowed character; in the Pear tree their 

 arrangement produces an irregular pentagon ; and 

 this is, also, the case in the Oak ; while in Laurel- 

 leaved Magnolia, M. grandiflora, the bundles are 

 seen at the distance of four or five diameters from 

 each other^ and projecting forwards, so as to seem 

 imbedded as it were in the pith. But, whatever 



