Horticulture Principles and Practices

Upper epidermis
Palisade layer
FIGURE 3–14 A transverse
section of a typical dicot leaf
showing internal structure.
Vascular bundle
Spongy mesophyll
Intercellular chamber
Lower epidermis
Stoma
Guard cells
FIGURE 3–15 Leaf venation
in a (a) monocot, (b) dicoty, (c) an
actual dicot leaf venation. (Source:
George Acquaah)
(a) Parallel venation
(b) Reticulate venation
(c) Reticulate venation
Stem
FIGURE 3–16
leaf.
A typical grass
Lamina
Sheath
more than one row of this tissue if the plant is exposed to intense sunlight. Palisade tissue
cells contain chloroplasts used in photosynthesis. Next to the lower epidermis is a
layer of widely spaced cells called spongy mesophyll. This tissue provides flexibility of
the lamina as it moves in the wind.
The vascular system of the stem extends to the leaf. Dicot leaves generally have a
single, large central vein, or midrib, from which secondary and tertiary veins branch out
into the lamina. This vascular system provides the framework of the leaf. The pattern of
veins (called venation) is a basis for classifying plants. Dicots have reticulate venation
(weblike), while monocots have parallel venation (Figure 3–15).
Leaves drop after a period of being on the stem. In dicots, a zone called the abscision
zone, located at the base of the petiole, is responsible for the dropping of leaves as they
age or as a result of adverse environmental conditions. The base of the petiole is a swollen
structure called a pulvinus, enlarged as a result of water imbibed by the parenchyma cells
in that region. Under moisture stress, the pulvinus cells lose water and collapse, resulting
in drooping of the petiole. When the pulvinus becomes turgid again, the leaf is lifted up.
Monocot leaves lack a petiole (Figure 3–16). Instead, they have a base (sheath) and
a lamina with parallel venation. The junction where the lamina attaches to the sheath may
have a structure called a ligule. An auricle or stipule may also be present.
3.4 Leaf 77