elements that support the stem. This group—which includes oak, ash, and
elm—is called ring porous(Fig. 1b). In the other group of hardwood trees,
the growth rings are more difficult to recognize because the vessels are
uniformly distributed throughout the tree ring, and the only demarcation
between successive layers is either a radial flattening of the last few ele-
ments formed or an increase in fibers near the end of the growth period.
This group oftrees is called diffuse porousand includes poplar, lime, and
beech (Fig. 1c). In the subtropics and tropics, there are no distinct growth-
ring zones (Fig. 1d), but trees sometimes form zonal layers, which are not
identical with real growth rings.
In addition to the differences in structure, the three groups differ
physiologically. In ring-porous wood, the latest growth ring fulfills the
major task of water conduction, and consequently a new ring must be
formed every year. In diffuse-porous woods and in conifer wood, previ-
ously formed growth rings participate in the water conduction. Hence,
under adverse climatic conditions, the trees do not need to form a growth
ring every year and may be characterized by absent or partially missing
rings. Conversely, it is possible that two growth increments may be formed
in a single year. These occurrences make the determination of growth40 Klein
a bcdFigure 1a–d
Photomicrographs (cross sections) of
(a)spruce wood (Picea abies); (b) oak wood
(Quercus petraea); (c) beech wood (Fagus sylvat-
ica); and (d) tropical wood (Hopea brachyptera).
Magnification 3 25.