Transcript Corn stem cross-section

The Plant Body
Apical Dominance
• Usually the growing terminal bud inhibits
the development of the lateral buds, a
phenomenon known as apical dominance as the distance between the shoot tip and
lateral buds, the influence of the apical
meristem lessens and the lateral buds
proceed with their development
Apical Dominance
Stem Function
The two main functions of the stems are conduction
and support
• Conduction involves moving substances
manufactured in the leaves through the phloem to
other parts of the plant including developing
leaves, stems, roots, developing flowers, seeds and
fruits and the xylem carries water from the roots to
the leaves, where water is transpired
• Support involves holding the plant off the ground supporting the principal photosynthetic organs of
the plant (the leaves) as well as flowers, seeds and
fruits
Stem Growth
• The organization of the apical meristem of the
stem is more complex than is the organization of
the apical meristem of the root
• The apical meristem adds cells to the plant body
and forms leaf primordia and bud primordia that
develop into lateral branches
• The apical meristem of a stem lacks a protective
cover like the root cap of roots
Stem Growth cont’d
• Protoderm always originates from the outermost
meristem cell layer
• Procambium and part of the ground meristem
(which will form the cortex and sometimes part of
the pith) form from the peripheral meristem
• The rest of the ground meristem (which forms
some or all of the pith) is formed by the pith
meristem
More Stem Growth
• Usually the meristematic activity causing the
elongation of the internodes is most intense at the
base of the developing internodes - if elongation
of the internodes occurs over a long period, the
meristematic base of the internode may be called
an intercalary meristem (a meristematic region
between two highly differentiated regions) –
intercalary meristems are very important in the
growth of grasses and grass-like plants
Three types of primary structure
1. In some conifers and dicots, the narrow,
elongated procambial cells (and
consequently the primary vascular tissues
that develop from them) appear as a more or
less continuous hollow cylinder within the
ground tissue - the outer region of the
ground tissue is called the cortex and the
inner region is the pith
Basswood stem cross-section
Three types of primary structure
2. In other conifers and dicots, the primary vascular
tissues develop as a cylinder of discrete strands
separated from one another by ground tissue
• The ground tissue separating the procambial
strands (and later mature vascular bundles) is
continuous with cortex and pith and is called the
interfascicular parenchyma (between the bundles)
• The interfascicular regions are often called pith
rays
Sunflower stem cross-section
Three types of primary structure
3. In most monocots and some herbaceous
dicots, the arrangement of procambial
strands and vascular bundles is more
complex - vascular tissues do not appear as
a single ring, but instead develop as more
than one ring or are scattered throughout the
ground tissue - here ground tissue cannot be
distinguished as pith and cortex - often
called pith
Corn stem cross-section
Twig structure
Leaf Traces
• At each node, one or more
vascular bundles diverge
from the cylinder of
strands in the stem, cross
the cortex and enter the
leaf or leaves attached at
that node
• The extensions from the
vascular system in the
stem toward the leaves are
called the leaf traces
• The wide gaps or regions
of ground tissue above the
level where leaf traces
diverge toward the leaves
are called leaf gaps
Plant habitat and leaf structure
• mesophytes - plants that require abundant soil
moisture and a fairly humid environment - the
most common plants - typically have fairly well
developed epidermis, especially on upper surface
of leaf, stomata on both sides of leaf
• hydrophyte - plants that depend on a very
abundant supply of water or which grow wholly or
partially submerged in water - have thin
epidermis, stomata only on upper surface
• xerophyte - plants adapted to arid habitats - very
thick epidermis, stomata open to stomatal crypts
with protective hairs
Dicot stomata
Red Oak Leaves
Shade Leaves
Sun Leaves