4.4 Leaf Development
4.4.1 Leaf Structure
Leavesare specialized structures responsible for most of the photosynthesis that takes place
in the plant, as well as functioning in respiration and transpiration. Leaves are initiated as
primordia from the shoot apical meristem as described earlier. As leaf primordia are speci-
fied by gene such as thePhangene, the abaxial (or top) and adaxial (bottom) surfaces
develop (Fig. 4.7a). Recall that leaves differ from the cotyledons in several ways including
the presence of the single-celled trichomes, or leaf hairs that function in the secretion of
various compounds that can attract or repel insects (Fig. 4.7b).
A cross section of a mature leaf shows the main cell types in the leaf (Fig. 4.7c). The
outer epidermal cell layers are derived from the L1 layer of the SAM in both monocots
and dicots and do not contain chloroplasts. The exception to this is thestomatal pore,
which is created from two guard cells that contain a specific number of chloroplasts,
depending on the ploidy of the plant. The interior leaf cells are filled with chloroplasts
that will autofluoresce when viewed under a fluorescent microscope. Dicot leaves have a
distinct dorsiventrality with an upper (adaxial) layer of oblong palisade cells, and a
lower (abaxial) layer of spongy mesophyll cells (both are derived from the L2 layer of
Figure 4.7.Leaf development. (a, b) Scanning electron micrographs of leaf primordia. Note the
presence of trichomes in (b). (c) Schematic of leaf cross section showing the different leaf cell
types. (d) Cross section through leaf vascular tissue. [Reprinted from Byrne (2006), with permission
from the Public Library of Science.]
96 PLANT DEVELOPMENT AND PHYSIOLOGY