The Inner Structure of the Plantlet. 33 
The age of a given part of the stem of a woody plant is ap- 
g g ! yP p 
proximately indicated by the number of its wood rings.* 
42. The Vital Part of 
Woody Stems in plants hav- 
ing more than one cotyledon 
(46) is limited to a compara- 
tively thin layer of bark and 
wood, of which the cambium 
forms the center. The cells of 
the so-called heart-wood, and 
those of the dry and furrowed 
outer bark, have lost their pro- 
toplasm, and so are no longer 
alive, though they serve a use- 
ful purpose in adding strength 
and protection to the vital layer. 
The heart-wood of a tree may 
largely decay without material- 
ly interfering with the vital pro- 
cesses. (Tig. 21). 
43. The Healing of 
Wounds. Cambium cells ex- 
posed to the air, by partial or 
complete removal of the bark, 
soon perish, hence growth 
ceases in a part of the stem 
thus injured. The uninjured 
cambium cells on the borders of 
bee 
as 
S 
‘= 
ic 
c 
A 
a 
=: 
Fig, 21. Live poplar tree with hol- 
low trunk, showing to what extent 
the heart-wood of a tree may decay 
without destroying its life. Madison, 
Wis. 
* More than one wood ring is sometimes formed in a season. If growth ceases 
during the summer from severe drought, or other cause, and is renewed later the 
same season, an extra ring is formed. 
