rl ORGANOGRAPHY. BOOK I. 



of Phytocrene gigantea, in which vessels exist, should prove to 

 belong to the medullary system. 



The vascular system in an Exogenous stem is confined to 

 the space between the pith and the bark, where it chiefly con- 

 sists of ducts, and vasiform or woody tissue collected into com- 

 pact wedge-shaped vertical plates (fig. 32. d), the edges of 

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

 contact with the medullary rays. 



That portion of the vascular system wliich 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 tliin 

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

 the vascular system of the stem in which spiral vessels are or- 

 dinarily found ; the whole of the vessels subsequently depo- 

 sited over the medullary sheath being ducts, or vasiform tis- 

 sue, with a few exceptions. The medullary sheath establishes 

 a comiection between the axis and all its appendages, the 

 veins of leaves, flowers, and fruits, being in all cases prolonga- 

 tions 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 wlien that of the surrounding parts has ceased. 



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 first, with which 

 it agrees exactly in structure, except that there is no medul- 

 lary sheath; the third year a third zone is formed on the 

 outside the second, in all respects like it; and so on, one 

 zone being deposited every year as long as the plant con- 



