_ woody tissue and spiral vessels, enclosing the pith, and term- 
_ ‘161. A new layer (consisting of a layer of cellular t 
_ it is pushed inwards, and becomes more compact by th 
_ eylindrical layers are separated at different points b: 
of elongated cellular t - 
pith towards the ci 
50 THE STEM. 
makes the principal bulk of the trunk and branches. It con- 
sists of concentric layers, each of which is composed of 
layer of woody tissue or of vasiform tissue, and ducts. At 
first there is only one layer of vascular tissue, consisting 0 
ed the medullary canal or sheath. In each succeeding year 
there are two layers formed, one of woody or vasiform tis- 
sue and one of vascular tissue, of which the latter is the ex- 
ternal. Hence the age of a stem of this kind may be 
by the number of concentric vascular cylinders in the of 
this is easily seen, as the layers of woody tissue which 
vene between the layers of ducts render the latter very 
tinct. 
159. The external woody layers next the bark are called 
the alburnum, and differ from the internal layers or true 
wood in being younger, softer, more succulent, and 
lighter colour. . 
160. The true wood is formed by the inner layers 
alburnum, which gradually acquire a greater degree of 
ness: the transition from alburnum to true wood is, h 
ever, almost imperceptible. 
and one of vascular tissue) is formed annually next the 
position of secretions and pressure of each succeeding 
nual layer, till at last it becomes almost solid, the sides 
the vessels and cells being gompressed; hence the 
hardness of such trees in the centre. Thus the wood 
such stems is exogenous, or growing from without. 
- 162. The fasciculi or bundles of vessels which comp 
