The Inner Structure of the Plantlet. 53 
The age of a given part of the stem of a woody plant is ap- 
proximately indicated by the number of its wood rings.* 
72. 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. (Fig. 21). 
isn: bee. Dealing 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 ,, 
Fig. 21. Live poplar tree with hol- 
w trunk, showing to what extent 
thus injured. The uninjured the heart-wood of a tree may decay 
: without destroying its life. Madison, 
cambium cells on the borders of wis. 
* More than one wood ring is sometimes formed ina 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. 
