the SAM). Vascular bundles containing xylem and phloem are present in the middle or L3-
derived layer. Monocot leaves vary but all contain a single photosynthetic cell type, the
mesophyll, and a specialized bundle sheath surrounding the vascular tissue. There are
many specializations of leaves, such as in xerophytic leaves, which are adapted to dry con-
ditions, which contain different cell types and arrangements of these cell types (Byrne
2006).
Mature leaves are often surrounded by a waxy, cuticle layer that provides protection and
prevents water loss. The epidermal cells secrete this layer and themselves provide protection
to the internal tissues. Since epidermal cells do not contain chloroplasts, they are essentially
colorless and facilitate the focusing of light to the active, photosynthetic mesophyll and
palisade cells below. The stomatal pores present in the epidermis allow for gas exchange
in photosynthesis and respiration, and are controlled by discrete signal transduction path-
ways that involve ABA, calcium, phosphatidic acid, and inositol-containing second
messengers. These signal transduction components are thought to eventually alter ion
channel activities that allow the guard cells to increase turgor, thus opening the stomatal
pore, or to decrease turgor, which results in stomatal closure. Thus, in addition to its role
in seed dormancy, ABA is also considered the drought-sensing hormone as its signal trans-
duction pathway can allow for stomatal closure, an important response to drought that
conserves water lost through transpiration.
It is interesting to note that most leaves contain more stomatal pores on their abaxial
surface than their adaxial surface. This location places them closer to the spongy mesophyll.
Indeed, the mesophyll layer within the leaf is the major site of photosynthesis in the plant,
and contains two cell types in dicots: the spongy mesophyll and the palisade mesophyll
cells. Both cell types are active in photosynthesis, yet have different shapes. It is thought
that the oblong shape of the palisade cells helps to further focus light on the spongy meso-
phyll cells. The gaps around spongy mesophyll are another adaptation that accommodates
the oxygen generated from photosynthesis.
4.4.2 Leaf Development Patterns
Besides photosynthesis, there are several interesting developmental considerations for
leaves. Leaf primordia first arise when a small group of cells on the outer edge of the
SAM gain leaf identity. These leaf primordia mature into a leaf bud utilizing a marginal
meristem to form the lamina or outer edge of the leaf, and a central meristem that gives
rise to the vascular tissue. Leaf buds can remain dormant in plants such as trees. Cell div-
ision within the leaf bud occurs at the base of the primordia or leaf, which means that cells
are pushed up toward the tip of the growing leaf. Along with cell division, cell expansion is
a critical process that produces large increases in leaf size. In general, cell expansion starts
after cell division has given rise to the main structure of the leaf. Thus, the younger the leaf,
the more active it is in cell division. Almost all mutants defective in the production of leaves
are also affected in the SAM, containing an under- or overcommitment to leaf primordia
cells. Another interesting characteristic of leaves is their placement on the plant, which is
calledphyllotaxy. Leaves are initiated in a precise pattern as the shoot meristem grows, pro-
ducing either alternate, opposite, tricussate (whorled), or spiral arrangements. In many
species, the number and position of leaves, or modified leaves such as the spines of a pine-
apple fruit, follow the Fibonacci number series (1,2,3,5,8,13,...). The venation pattern of
leaves also varies with monocots containing parallel veination, while most dicot leaves have
a reticulate pattern.
4.4. LEAF DEVELOPMENT 97