ANATOMY OF TISSUES. 47 
where cells derived from isodiametric meristem afterward grow 
long and become prosenchymatic. . 
If there are more than two of these cambium clusters, which 
is the case in the greater number of dicotyledonous stems, they 
are arranged in a circle about the stem axis, and are surrounded 
in all cases by parenchymatic lasting tissue. When they assume 
the function of meristem they form new walls, mostly parallel 
with the surface of the stem. It may also be noticed at this 
stage that they are more or less regularly arranged in tangential 
rows, and that the new cells are cut 
off both toward the centre and the cir- 
cumference. We find these clusters of 
cambium developing new lasting tissue 
on two sides, or radially. In this way 
they become tangential: centres of 
growth, isolated from each other by 
the intervening parenchyma. Each 
one of these cambium clusters with 
the cells derived from it forms what 
is known as a vascular bundle. Fig. 23. 
Diagrammatic sketch, cross-sec- 
: tion of herbaceous dicotyledon. 
any length of time, it will be readily = g ground tissue. m pith. ¢ 
vascular bundles. —( Wiesner.) 
If this erowth is to continue for 
seen that.some corresponding change 
must take place in the cells of the lasting tissue lying between 
the bundles. If these cells are not to be torn apart and broken 
by the strain, they must either grow in size to accommodate 
themselves to the increasing radius, dr they must themselves 
become meristematic and so originate new cells. This leads to 
the discussion of a new kind of meristem, which may be called 
secondary, to distinguish it’: from that already described as 
primary. 
It is one of the characteristics of thin-walled living cells, 
that they may become fully .differentiated, and take on the 
properties of lasting tissue, and afterward assume the function 
