Agriculture Reference
In-Depth Information
Leaves, canopies and light
interception
Leaf anatomy and morphology
Leaf primordia are initiated by periclinal divisions in layers
or
of the tunica
on the flanks of apical meristems (Pratt,
). They develop into protuber-
ances flattened on the adaxial side. The base of the leaf primordium is an
intercalary meristem and forms the petiole. The leaf blade develops concomi-
tantly from two layers of cells derived from marginal and sub-marginal initials
which produce a marginal meristem on the lateral flanks of the midrib. The
superficial cells of the marginal meristems divide anticlinally to form epider-
mal cells. The sub-epidermal cells on the adaxial (upper) side become the first
row of palisade cells and those of the abaxial side form the spongy mesophyll.
Cells between these two layers form the central mesophyll and the smaller vas-
cular bundles. On the adaxial side sub-epidermal and central meristematic
cells differentiate into one to three or more layers of palisade cells depending
on leaf type and environmental conditions. Each mature palisade cell is sur-
rounded by air space continuous with that in the spongy mesophyll, except in
the vicinity of a vein.
At maturity (Figure
) the adaxial cuticle consists of a layer of wax and
cutin. The abaxial epidermal cells have thinner cuticles than the adaxial
ones. They are variable in thickness and shape except for the paired, kidney-
shaped guard cells which are nearly constant in size within a cultivar and
surround the pores or stomates through which gas exchange with the exter-
nal air takes place. Initially the stomata are immature and non-functional
with no evidence of a pore, but even under English conditions leaves on the
flower clusters can have a high proportion of functional stomata in April
and May, and by June all stomata are functional (Slack,
.
). Increase in
turgor causes the thin guard cell walls adjacent to the epidermal cells to
stretch and the elliptical pore opens. Stomata in apple and pear are found
only on the lower, abaxial, leaf surface and, contrary to earlier studies, Slack
(
) found no systematic distribution pattern over this surface and a mean
