Agriculture Reference
In-Depth Information
four different vascular plant lineages evolved leaves: the ferns, sphenopsids, progym-
nosperms, and seed plants (Boyce and Knoll 2002). The leaves of extant members of
these lineages are the primary photosynthetic organs in the great majority of plant
species. The earliest leaves in all four lineages were small, narrow, and single veined
(“microphylls”), arrayed along highly dissected branching systems but larger and
broader multiveined leaves (“macrophylls”) gradually become predominant in the
fern, gymnosperm, and angiosperm lineages (Boyce and Knoll 2002). The earliest of
these land plants are believed to have been evergreen, but by the early Carboniferous
Archaeopteris
may have had some deciduous characteristics (Addicott 1982; Thomas
and Sadras 2001). The unambiguous origin of a seasonally adapted deciduous habit
arose only later in the polar forests of a “greenhouse Earth” where plants had to con-
tend with dark but warm winters and fire-prone conditions (Brentnall et al. 2005). By
the Permian there is some evidence for the seasonally programmed turnover of leaves
in the
Glossopteris
flora of polar regions (Taylor and Ryberg 2007) and strong evi-
dence for deciduous polar forests by the Cretaceous (Taggart and Cross 2009).
The leaves of contemporary plant species typically are arrayed along a stem segment
to form a shoot. The basic unit of shoot construction is a metamer consisting of a
leaf and bud at a node along a stem and an associated internodal stem segment
(Barlow 1989). Shoots composed of some number of metamers (Fig.
2.1
) can be
considered the modular units of organization in the aboveground portion of plants
d
c
b
a
Fig. 2.1
Growth of a plant by the accumulation of modules. (
a
) The shoot, a stem section with
leaves, is the basic modular unit of plant vegetative growth. (
b
) A plant canopy grows by accumu-
lation of modules, sometimes only by apical extension, and other times (
c
) by lateral branching
from dormant buds. Some leaves and shoots typically will be shed as growth of the entire plant
proceeds, and the developing plant canopy can take on a degree of asymmetry as shoots interact
with one another and respond to their immediate microenvironment (
d
)
