100 PLANT LIFE 
may be allowed, it does not seem to matter 
much how it is disposed. 
Every one must have noticed that the great 
majority of our shrubs and branching trees 
increase in girth as they get older. This in- 
crease is produced by a specially active layer 
of ‘“‘embryonic’”’ tissue known as cambium 
(Fig. 11c), which forms a cylindrical sheet of 
cells situated at the outer limit of the wood, 
which it thus completely encloses. By the 
active division of this cambium the cylinder 
or zone of young cell tissue is temporarily 
rendered thicker every year, and then the 
layers of cells which abut on the existing 
wood are themselves differentiated into xylem, 
to form the new annual ring of wood which 
is added every year to the wood of the trunk. 
A few of the outermost layers of the cylinder 
are similarly transformed into bast or phloém, 
and only a thin cell layer now remains as 
a cylindrical sheet of cambium which still 
continues to separate the wood and bast. 
Next year this again increases in thickness, 
and the new layers thus produced go through 
the same changes as before. 
In this way the annual rings of wood are 
produced which are seen when tree trunks 
are sawn across. It is due to the still un- 
differentiated and relatively thick sheet of 
young cells produced every spring that the 
‘‘bark”’? is so easily separated from the 
wood at this season. For the walls are thin 
and the cells are rich in protoplasm and cell 
