198 MEDULLARY SHEATH WOOD. [BOOK i. 



The VASCULAR SYSTEM in an Exogenous stem is confined 

 to the space between the pith and the bark, where it chiefly 

 consists of ducts, and pitted or woody tissue collected into 

 compact wedge-shaped vertical plates, jig. 3L the edges of 

 which rest on the pith and bark, and the sides of which are 

 in contact with the medullary rays. It consists of liber, 

 ivood, and the medullary sheath. 



* Medullary Sheath. 



That portion of the vascular system which is first generated 

 is in immediate contact with the pith, to which it forms a 

 complete sheath, interrupted only by the passage of the 

 medullary rays through it. It consists of spiral vessels and 

 woody tissue intermixed, and forms an exceedingly thin 

 layer, called the medullary sheath. This is the only part of 

 the vascular system of the stem in which spiral vessels are 

 ordinarily found; the whole of the vessels subsequently 

 deposited over the medullary sheath being bothreiichyma- 

 tous tissue, with a few exceptions. The medullary sheath 

 establishes a connection between the axis and all its appen- 

 dages, the veins of leaves, flowers, and fruits, being in all 

 common cases prolongations of it. It has been remarked by 

 Senebier, and since by De Candolle, that it preserves a green 

 colour even in old trunks, which proves that it still continues 

 to retain its vitality when that of the surrounding parts has 

 ceased. 



** Wood. 



The vascular system of a stem one year old consists of a 

 zone of wood lying between the pith and the bark, lined in 

 the inside by the medullary sheath, and separated into 

 wedge-shaped vertical plates by the medullary rays that pass 

 through it. All that part of the first zone which is on the 

 outside of the medullary sheath is composed of woody tissue 

 and vessels intermixed in no apparent order ; but the vessels 

 are generally either in greater abundance next the medullary 

 sheath, or confined to that side of the zone, and the woody 

 tissue alone forms a compact mass on the outside. The 

 second year another zone is formed on the outside of the 



