359
the stomatal (or substomatal) cavity. The stomata open and close, regulated by turgor pressure, to allow gases, including water vapor, C dioxide, and oxygen, to
exchange between the outside air and the stomatal cavity. Because the anatomical
structures of species are different across the interior of the leaf, their biochemistry
is also specialized, causing the asymmetric distributions of reflectance and absorption patterns as light passes through the leaf, shown in Fig. 14.4, for photosynthetic
light at two solar zenith angles entering from the lower side of the leaf.
Xerophytes and hydrophytes illustrate the extremes of leaf adaptations for cuticle characteristics. Aquatic species that float on the surface like water hyacinth
(Eichhornia crassipes) and water lily (Nymphaea sp.) have stomata on the upper,
adaxial side that is open to the atmosphere. Submerged aquatic species, like
Brazilian waterweed (Egeria densa), hydrilla (Hydrilla verticillata), and Eurasian
watermilfoil (Myriophyllum spicatum), lack stomata or they are nonfunctional, and
their cuticle is thin and reduced to allow direct gas exchange with the water. These
species typically have other morphological traits that support adaptation to the
aquatic habitat, including very small leaves. In contrast, xerophytes like semiarid
grasses, e.g., the beach grass Ammophila breviligulata, and conifers or succulent
species like agaves and cacti, have stomata in deep pits that reduce transpiration by
retaining high vapor pressure in the cavity. In addition, other traits typically present
in xerophytes include thick cuticles, pubescence, and a reduced stomatal complex.
Leaf traits such as these tend to be clustered, representing a suite of adaptive traits;
thus, to identify a taxon, we expect several traits to be present in particular configurations; thus, potential identification is enhanced by patterns of traits (see Serbin
and Townsend, Chap. 3; Morsdorf et al., Chap. 4; Bolch et al., Chap. 12).
14.7.1 Surface Characteristics of Epidermal Cells
Epidermal cells can be coated with smooth wax, which enhances specular scattering
off the surface of leaves. This type of surface scattering occurs when the leaf is
oriented to cause forward scattering of incoming beam. Light specularly scattered
from a leaf surface has the same wavelength composition and intensity as the incoming beam, so it does not provide any information about the interior of the leaf.
Smooth, waxy leaves having shiny, glabrous surfaces are often found in young
leaves of broadleaf shrubs, trees, or herbaceous understory species. In woody plants,
this is generally a mechanism to avoid absorbing excess photosynthetically active
radiation under high light conditions. Species that have this trait include members
of Cinnamomum in the laurel family (e.g., Cinnamomum camphora and C. parthenoxylon) and Magnolia grandiflora, which are found in warm subtropical habitats, often in the understory.
The waxy cuticle or outer layer of the epidermis produces a 3-D structure of
waxes and cutin of variable thickness and cell types that create a diverse range of
textured surfaces and colors. Leaf traits like thick cuticles are common to a wide
range of plants such as columnar cacti (e.g., Pilosocereus leucocephalus) and
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
the stomatal (or substomatal) cavity. The stomata open and close, regulated by turgor pressure, to allow gases, including water vapor, C dioxide, and oxygen, to
exchange between the outside air and the stomatal cavity. Because the anatomical
structures of species are different across the interior of the leaf, their biochemistry
is also specialized, causing the asymmetric distributions of reflectance and absorption patterns as light passes through the leaf, shown in Fig. 14.4, for photosynthetic
light at two solar zenith angles entering from the lower side of the leaf.
Xerophytes and hydrophytes illustrate the extremes of leaf adaptations for cuticle characteristics. Aquatic species that float on the surface like water hyacinth
(Eichhornia crassipes) and water lily (Nymphaea sp.) have stomata on the upper,
adaxial side that is open to the atmosphere. Submerged aquatic species, like
Brazilian waterweed (Egeria densa), hydrilla (Hydrilla verticillata), and Eurasian
watermilfoil (Myriophyllum spicatum), lack stomata or they are nonfunctional, and
their cuticle is thin and reduced to allow direct gas exchange with the water. These
species typically have other morphological traits that support adaptation to the
aquatic habitat, including very small leaves. In contrast, xerophytes like semiarid
grasses, e.g., the beach grass Ammophila breviligulata, and conifers or succulent
species like agaves and cacti, have stomata in deep pits that reduce transpiration by
retaining high vapor pressure in the cavity. In addition, other traits typically present
in xerophytes include thick cuticles, pubescence, and a reduced stomatal complex.
Leaf traits such as these tend to be clustered, representing a suite of adaptive traits;
thus, to identify a taxon, we expect several traits to be present in particular configurations; thus, potential identification is enhanced by patterns of traits (see Serbin
and Townsend, Chap. 3; Morsdorf et al., Chap. 4; Bolch et al., Chap. 12).
14.7.1 Surface Characteristics of Epidermal Cells
Epidermal cells can be coated with smooth wax, which enhances specular scattering
off the surface of leaves. This type of surface scattering occurs when the leaf is
oriented to cause forward scattering of incoming beam. Light specularly scattered
from a leaf surface has the same wavelength composition and intensity as the incoming beam, so it does not provide any information about the interior of the leaf.
Smooth, waxy leaves having shiny, glabrous surfaces are often found in young
leaves of broadleaf shrubs, trees, or herbaceous understory species. In woody plants,
this is generally a mechanism to avoid absorbing excess photosynthetically active
radiation under high light conditions. Species that have this trait include members
of Cinnamomum in the laurel family (e.g., Cinnamomum camphora and C. parthenoxylon) and Magnolia grandiflora, which are found in warm subtropical habitats, often in the understory.
The waxy cuticle or outer layer of the epidermis produces a 3-D structure of
waxes and cutin of variable thickness and cell types that create a diverse range of
textured surfaces and colors. Leaf traits like thick cuticles are common to a wide
range of plants such as columnar cacti (e.g., Pilosocereus leucocephalus) and
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…