130 ELEMENTS OF PLANT ANATOMY. 
The secondary parenchymatic and living element of the 
wood is the medullary ray cell which also forms a part of the 
phloem elements. The origin of the primary ray has been 
given and its function referred to. It is the only one extend- 
ing from pith to rind and so connecting these portions. As 
each successive year adds a new zone of growth, the distance 
between these rays is constantly widening toward the circum- 
ference, and a new supply of living cells is added by the forma- 
tion of new medullary rays. The cambium cells which are about 
to produce these divide by walls at right angles to their long 
axes, thus forming a parenchymatic meristem. The rays so 
formed are named secondary, or rays of the same order as the 
year in which they arise. 
Regarding the character of the separate cells, their shape is 
similar to that of the wood parenchyma; their position, however, 
5 is reversed, so that the long axis 
is at right angles with that of 
the stem. Their walls are rather 
thick, unlignified, and richly 
supplied with simple pores. As 
a rule the primary rays have a 
greater width in the cross-section 
than the secondary; that is, 
they are often two, three, and 
even more cells wide, while the 
secondary are often only a single 
Co cellin width. The depth of the 
FIG. 55. ray varies with the habit of the 
Fon 
As 
eo 
Diagrammatic cross-section through a three plant. Some are the depth of 
years’ dicotyledonous stem (Type Ber- 3 ; 
beris). 2 xylem. p phloem. ec cambium. only a single cambium cell 
i interfasicular cambium. i pith. m others or several. 
pith crown. M primary medullary rays. : 
s secondary medullary rays of the first ; A cross-section of a stem of 
s' of the second order. — ( Wiesner.) ’ 
several years’ growth presents 
the appearance of a circle composed of narrow, wedge-shaped 
