92 
ORGANOGRAPHY. 
BOOK I. 
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 increase 
outwardly, continuing to maintain the connection between 
the centre and the circumference. Botanists call them me- 
dullary rays (or. plates ) ; and carpenters, the silver grain. 
They are composed of muriform cellular tissue (Plate I. 
fig. 7.), often not consisting of more than a single layer of 
cellules ; but sometimes, as in Aristolochias, the number of 
layers is very considerable. In horizontal sections of an Ex- 
ogenous stem, they are seen as fine lines radiating from the 
centre to the circumference ; in longitudional sections they 
produce that glancing satiny lustre which is in all discover- 
able, 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, un- 
less those curious plates described by Griffith in the wood 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, \vhere it chiefly 
consists of ducts, and pitted or woody tissue collected into 
compact wedge-shaped vertical plates, Jig. 34. 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 wffiich 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 medidlary 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 de- 
posited over the medullary sheath being bothrenchymatons 
tissue, with a few exceptions. The medullary sheath esta- 
blishes a connection between the axis and all its appendages, 
the veins of leaves, flowers, and fruits, being in all cases pro- 
longations of it. It has been remarked by Senebier, and since 
by De Candolle, that it preserves a green colour even in old 
