Transpiration and the Ascent

236 7. Transpiration
blocked by gas bubbles, tyloses, or deposits of gum, resin, and other substances.
In older stems most of the cross section is composed of dead heartwood, which
usually is darker in color and denser than the sapwood. The lack of essential
physiological function in heartwood is indicated by the survival of hollow trees
in which it has decayed and disappeared.
Xylem of woody angiosperms may be either ring or diffuse porous. In ring
porous species such as ash, oak, and elm the first xylem vessels formed each
spring (earlywood) are much larger in diameter than those formed late in the
preceding summer (latewood), resulting in a sharp demarcation between Successive
annual rings. In diffuse porous trees such as birch, maple, and poplar the
vessels usually are smaller in diameter, and those formed early in the growing
season (earlywood) are nearly the same diameter as those formed at the end of
the previous growing season (latewood). The difference between early and latewood
enables observers to determine the number of annual rings, and differences
in their width indicate the occurrence of droughts and other conditions
unfavorable to growth. This relationship led to the development of dendrochronology,
the study of the width of rings of old trees and timbers of old buildings,
both as an indicator of past rainfall and of age of buildings (Fritts, 1976;
Kozlowski et at., 1991, p. 66).
Interest in the conducting system of trees developed long ago. According
to Zimmermann (1978), Leonardo da Vinci (d. 1519) wrote that when all
branches of a tree are put together they are equal in thickness (area) to the trunk
below them. This was developed into the pipe model by Shinozaki et al.
(1964a,b) who treated the xylem as a collection of tubes. Zimmermann (1978)
has an interesting discussion of the varying specific conductivity in various
parts of the conducting system, examples of which are shown in Fig. 7.14 ..The
relationship between structure and water movement was reviewed in detail by
Fiscus and Kaufmann (in Stewart and Nielsen, 1990) and by Tyree and Ewers
(1991), and the structure of woody stems was discussed in Kramer and
Kozlowski (1979, pp. 18-38).
In many ring-porous species, water movement is confined to one or a few
outer rings because older vessels and tracheids are blocked by air bubbles (embolisms),
tyloses caused by extrusion of protoplasts through cell wall pits into
the cavities of neighboring cells, and deposits of gum and resin. In diffuseporous
species, several annual rings may function in water movement. The resistance
to water flow is much higher in conifer stems where water moves
through tracheids with numerous cross walls than in deciduous, broadleaf trees,
and the resistance to longitudinal flow is relatively low in vines and roots. The
specific conductivity or conductivity per unit of leaf weight in various parts of a
birch tree is shown in Fig. 7.14 and the rate of flow in various parts of an oak
tree can be seen in Fig. 7.18. According to Tyree and Sperry (1989) the specific
conductivity is low in twigs and petioles and the chief resistance to water move-