358 CONSERVATIVE ORGANS. [LECT. VII* 



first noticed by Dr. Hill, who named it Corona, 

 yet, it has been overlooked or confounded with 

 the wood, or the alburnum, by almost every suc- 

 ceeding British Phytologist, until Mr. Knight's at- 

 tention was directed to it in searching for his cen- 

 tral vessels. It is readily distinguished, in either a 

 transverse or a longitudinal section of many stems, 

 by its green colour, which appears deeper as con- 

 trasted with the dead white, the more usual hue of 

 the pith which it surrounds ; but it is also easily 

 traced in the succulent dicotyledonous stem as 

 soon as it is evolved from the seed, separating the 

 pith from its herbaceous investiture. 



When viewed under the microscope the me- 

 dullary sheath appears to be composed of a cel- 

 lular substance, in which are imbedded longi- 

 tudinal layers of spiral tubes *. It is not easy 

 to comprehend the meaning of Mr. Knight, when 

 he speaks of another description of vessels as 

 being found here, to which he says " the spiral 

 " vessels are every where appendages," and which 

 he names central vessels, " to distinguish them 

 " from the spiral tubes and the common tubes of 

 " the wood-f~." In the stems which I have exa- 



* Vide Plate 6, fig. 7. in which the space from d. to e. re- 

 presents the appearance of the medullary sheath in a longitudi- 

 nal section of an annual twig of Horse Chesnut ; 7. 7. 7. the 

 bundles of spiral vessels ; and 8. the parenchyma, or green cel- 

 lular lining of the sheath. 



f Phil. Trans. 1801. 



