The SAM is the control center of the plant and directs the development of all aboveground
differentiated tissues such as the stems, leaves, thorns, flowers, and fruits. Cells within mer-
istems undergo cell division quickly, and are usually smaller because they have smaller
vacuoles than differentiated plant cells (Fig. 4.1).
The root also contains a similar control center, theroot apical meristem(RAM) that
functions in generating new root cells within the root tip (Fig. 4.1). A section through
the root shows that roots are often full of starch granules that can be visualized by staining
with potassium iodide, which turns starch a blue-brown color. One can also see the meris-
tematic zone at the root tip, the root cap, a protective covering, and the ordered files of cells
resulting from the root initial cells within the root apical meristem. One may also be able to
view the quiescent center (QC), so called because cells are “sleeping” or slow to undergo
cell division.
Axillary budsare the third type of meristems that give rise to new tissues. Axillary buds
may be found on stems, and under the right conditions can give rise to new shoot apical
meristems.
Plant cells within shoots and roots are organized into specialized tissues that enable
the organism to carry out necessary functions. The tissue systems of plants are the
dermal, vascular, and ground tissue systems. Thedermalsystem is composed of the epider-
mal, or outermost, cell layer, which covers the entire plant. Thevasculartissue system
is composed of the xylem, phloem, and other conducting cells that transport water and
nutrients. This tissue is present in most plant tissues, but can be arranged differently
within each organ. Theground tissueis composed of the cells in between the epidermis
and the vascular tissue.
There are many different specialized plant organs. In addition to the shoot and root
apical meristems, most angiosperms contain stems, leaves, lateral roots, and reproductive
tissues such as flowers and their component tissues (anthers, filaments, pollen, etc.).
Each of these tissues can impact the development and physiology of the plant, and as
such must be considered when manipulating gene expression in transgenic plants.
Specific considerations for each of these tissues will be discussed as we chart the develop-
ment and physiology of an average plant in the succeeding sections.
4.2 Embryogenesis and Seed Germination
4.2.1 Gametogenesis
The lifecycle of flowering plants alternates between a haploid organism, the gametophyte,
and a diploid organism, the sporophyte. Plants have male and female gametophytes, both of
which are multicellular and are produced within the flower (Fig. 4.2). The mature male
gametophyte, the pollen grain, has three cells: a vegetative cell and two 1Nsperm cells.
Pollendevelopment (Fig. 4.3) occurs in theanther, which is a specialized structure of
the flower, with the meiotic divisions of the microsporocytes to form a tetrad of haploid
spores. The microspores are embedded in callose, and release from the tetrad requires
enzymes secreted by somatic cells in the anther. Mature pollen grains have complex
walls with two layers, the inner intine and the outer exine layer.
Self-incompatibilityas a mechanism to limit reproduction was discussed in Chapter 2.
However, fertilization also depends on the gene products that are required for normal devel-
opment of the pollen andovules. Scientists have identified several of these required gene
products by taking a genetic approach. To identify molecules involved in either
4.2. EMBRYOGENESIS AND SEED GERMINATION 85