STRUCTURE.] CAMBIUM MEDULLARY RAYS. 197 



in all the living parts of plants, and out of which new organs 

 are formed. (See p. 7). It is, therefore, here introduced in 

 connection with bark, merely in compliance with old custom. 

 Its history has been elaborately investigated by De Mirbel, 

 to whose papers in the Comptes rendus, Annales des Sciences, 

 and Archives du Museum, the reader is referred. There is 

 also a short abstract of one of his papers in the Annals of 

 Natural History, vol. vi. p. 330. 



***** Medullary Rays. 



The cellular system of the pith and bark are, in the 

 embryo and youngest shoots, in contact; but the woody 

 system, as it forms, gradually interposes between them, till 

 after a few weeks they are distinctly separated, and in very 

 aged trunks are sometimes divided by a space of several feet ; 

 that is to say, by half the diameter of the wood. But what- 

 ever may be the distance between them, a horizontal com- 

 munication of the most perfect kind continues to be 

 maintained. When the woody system is first developed 

 in the cellular system, separating the pith from the cor- 

 tical integument, it does not completely separate them, but 

 pushes aside a quantity of cellular tissue, pressing it tightly 

 into thin vertical radiating plates : as the woody system 

 extends, these plates grow outwardly, continuing to maintain 

 the connection between the centre and the circumference. 

 Botanists call them medullary rays (or plates] ; and car- 

 penters, the silver grain. They are composed of muriform 

 cellular tissue (Plate I. fig. 7.), often not consisting of more 

 than a single layer of cells ; but sometimes as in Birthworts, 

 (Aristolochia) , the number of layers is very considerable. In 

 horizontal sections of an Exogenous stem, they are seen as 

 fine lines radiating from the centre to the circumference ; in 

 longitudinal sections they produce that glancing satiny lustre 

 which is in all discoverable, and which gives to some, such as 

 the Plane and the Sycamore, a character of remarkable beauty. 



No vascular tissue is ever found in the medullary rays, 

 unless those curious plates described by Griffith in the wood 

 of Phytocrene gigantea, in which vessels exist, should prove 

 to belong to the medullary system. 



