The Questions of Developmental Biology

When the shoot emerges from the soil, most of the sporophyte body plan remains to be
elaborated. Figure 20.20 shows the basic parts of the mature sporophyte plant, which will emerge
from meristems.
Meristems
As has been mentioned, meristems are clusters of cells that allow the basic body pattern
established during embryogenesis to be reiterated and extended after germination. Meristematic
cells are similar to stem cells in animals.* They divide to give rise to one daughter cell that
continues to be meristematic and another that differentiates. Meristems fall into three categories:
apical, lateral, and intercalary.
Apical meristems occur at the growing shoot and root tips (Figure 20.21). Root apical meristems
produce the root cap, which consists of lubricated cells that are sloughed off as the meristem is
pushed through the soil by cell division and elongation in more proximal cells. The root apical
meristem also gives rise to daughter cells that produce the three tissue systems of the root. New
root apical meristems are initiated from tissue within the core of the root and emerge through the
ground tissue and dermal tissue. Root meristems can also be derived secondarily from the stem of
the plant; in the case of maize, this is the major source of root mass.
The shoot apical meristem produces stems, leaves, and reproductive structures. In addition to the
shoot apical meristem initiated during embryogenesis, axillary shoot apical meristems (axillary
buds; see Figure 20.20) derived from the original one form in the axils (the angles between leaf
and stem). Unlike new root meristems, these arise from the surface layers of the meristem.
Angiosperm apical meristems are composed of up to three layers of cells (labeled L1, L2, and L3)
on the plant surface (Figure 20.22). One way of investigating the contributions of different layers
to plant structure is by constructing chimeras. Plant chimeras are composed of layers having
distinct genotypes with discernible markers. When L2, for example, has a different genotype than
L1 or L3, all pollen will have the L2 genotype, indicating that pollen is derived from L2.
Chimeras have also been used to demonstrate classical induction in plants, in which, as in animal
development, one layer influences the developmental pathway of an adjacent layer.
The size of the shoot apical meristem is precisely controlled by intercellular signals, most likely
between layers of the meristem (reviewed by Doerner 1999). Mutations in the Arabidopsis
CLAVATA genes, for example, lead to increased meristem size and the production of extra
organs. STM has the opposite effect, and double mutant phenotypes are consistent with the
hypothesis that the two work together to maintain meristem size (Clark et al. 1996). Perhaps they
balance the rate of cell division (which enlarges the meristem) and the rate of cell differentiation
in the periphery of the meristem (which decreases meristem size) (Meyerowitz 1997).
Lateral meristems are cylindrical meristems found in shoots and roots that result in secondary
growth (an increase in stem and root girth by the production of vascular tissues). Monocot stems
do not have lateral meristems, but often have intercalary meristems inserted in the stems
between mature tissues. The popping sound you can hear in a cornfield on a summer night is
actually caused by the rapid increase in stem length due to intercalary meristems.
Root development