Transpiration and the Ascent

218 7. Transpiration
Leal Orientation. Leaves with surfaces perpendicular to the sun's rays are
warmer than those more Or less parallel, and are likely to have a higher rate of
transpiration. Leaves of many plants tend to be more or less perpendicular to
midday radiation, but there are exceptions such as some species of Lactuca and
Silphium, turkey oak (Q. laevis), and jojoba (Simmondsia chinensis). The
leaves of Stylosanthes humilis normally are perpendicular to the sun's rays but
when the pulvini lose turgor they become parallel. Pine needles that OCCurin
bundles, such as those of the three and five needle pines, shade one another and
reduce transpiration and photosynthesis per unit of surface (Kramer and Clark,
1947). Leaf orientation may be less important in stands of plants than in individual
plants.
Leaf Surface Characteristics. The outer surfaces of leaves, flower petals,
stems, and fruits often are covered by a relatively waterproof layer, the cuticle.
This is composed of cutin and wax, anchored to the epidermis by a layer of
pectin (Fig. 7.7). When dry, the cuticle is relatively impermeable to water, as
shown by its high resistance (Table 7.4), but when wet it is fairly permeable to
water vapor and a variety of solutes (Sch6nherr, 1976). The cuticle generally is
thinner on shade than on sun leaves, and thinner on leaves grown with adequate
water than on leaves subjected to water deficits (Bengston et at., 1978). Cuticle
mutants have been found in sorghum, some of which displayed thin cuticles and
high water loss through the cuticle (Jenks et at., 1994). Permeability to water
must depend more on the amount of wax than on the thickness of the cuticle
because cuticular transpiration often increases severalfold when the wax is re-
cuticle
birefringent wax
-embedded in cutin
Figure 7.7 Diagram of an upper surface of an epidermal cell of pear leaf, showing cuticle, composed
of cutin and wax" The cutin is anchored to the cellulose wall by a layer of pectin" From
Kramer (1983), after Norris and Bukovac (1968),