72 
OnGANOGRAPHY. 
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. c?), 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 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 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 connection 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 when 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- 
