88 CAMBIUM LAYER.—MEDULLARY RAYS. 
seen, a layer of vitally active cells is placed, to which the name 
of cambium-layer or cambiwm has been given. It is from these 
cambium cells that the new layers of wood and phloém are 
formed, and from the fact of the cambium-layer being situated 
between the xylem and the phloém of the indefinite fibro-vas- 
cular bundles of which Exogenous stems are composed, it follows 
that the layers of increase to these parts of the bundle are in 
continuity with the previous layers. The cells composing the 
cambium-layer are of a very delicate nature, and consist of a 
thin wall of cellulose, containing a nucleus, protoplasm, and 
watery cell-sap ; in fact, they contain all the substances which 
are present in young growing cells. These cells, from their be- 
coming changed into the matured woody tissues and phloém, were 
called cambium-cells, hence the origin of the names cambiwm and 
cambium-layer applied to this portion of the stem. This layer 
is dormant during the winter, at which time the bark is firmly 
attached to the wood beneath, but it is in full activity in the 
spring, when it becomes charged with the materials necessary 
for the development of new structures, and then the bark may 
be readily separated from the wood beneath, but such separa- 
tion can only be effected by the rupture of the cells of which it 
is composed. The cambium layer is called a formative or gene- 
rating tissue, or meristem, because its component cells are capable 
of dividing and forming permanent tissue, or that in which the 
cells have ceased to divide, and have assumed their definite form. 
5. Medullary Rays.—We have already seen that at first the 
stem consists entirely of parenchyma, but that in a short time 
fibro-vascular portions are developed, so that at the end of the 
first year’s growth, in consequence of the development of the 
wood and phloém, this parenchyma becomes separated into two 
regions—an internal or pith, and an external forming the cel- 
lular layers of the bark ; the separation, however, not being com- 
plete, but the two being connected by tissue of the same nature 
as themselves, to which the name of medullary rays has been 
applied (jigs. 176, b, and 182, r). 
The cells forming these medullary rays, like those of the 
pith, are part of the fundamental tissue of the stem (page 81) ; 
but, unlike the cells of the pith, which remain of a more or 
less rounded form, they differ from them in form, and become 
much flattened in a radial direction (jigs. 94, and 185, B, 7, 7), 
owing to the pressure which the neighbouring wedges of the 
wood have exerted upon them. As new rings of wood are 
formed in successive years, fresh additions are made to the 
ends of the medullary rays from the cambium, so that, how- 
ever large the space between the pith and the cellular layers 
of the bark ultimately becomes, the two are always kept in 
connexion by their means. Besides the medullary rays which 
thus extend throughout the entire thickness of the{wood, others 
are also commonly developed between them in each succeeding 
