The Plant Body
Fun plant facts
• There are over 270,000 species of plant - they differ from
each other in light gathering shoots and water/nutrient
gathering roots, but they also differ from each other in
terms of biochemistry
• Because plants can’t move they have difficulty
reproducing and evading predators
• Plants produce many different chemicals to prevent or at
least minimize animal predators - many of these defensive
chemicals are used by us for medicines – 80% of all
people get their medicines directly from plants - 25% of
our prescription medicines come from plant chemicals
Commonly used and abused
chemicals substances from plants
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caffeine in coffee
theophylline in tea
theobromine in chocolate
nicotine in tobacco
cocaine from coca plant leaves
THC in Cannabis
and many other narcotic and psychoactive
substances
Cooksonia – 408 MYA
Basic plant structures
Plant Tissue Types
• Meristematic tissue - site of growth in plant;
origin of the other tissue types
• apical meristems - site of primary growth; lateral
meristems - site of secondary growth
• Dermal tissue system - the outer protective
covering of the plant
• Vascular tissue system - comprises the xylem and
phloem - it is embedded within the ground tissue
system – xylem forms wood in woody plants
• Ground tissue system - the inner supportive
tissues of the plant – pith in herbaceous plants
Primary Plant Growth
• Primary growth originates in the apical
meristems and results in increases in length
- tissues originating from primary growth
make up the primary plant body (primary
xylem, primary phloem, etc) - many
vascular plants consist entirely of primary
tissues
Secondary Plant Growth
• Secondary growth originates in the lateral
meristems and results in increases in width - there
are two lateral meristems
• Vascular cambium which produces secondary
vascular tissues - secondary xylem to the inside
and secondary phloem to the outside
• Cork cambium is outside the vascular cambium it forms periderm, which is made of cork tissue the periderm replaces the epidermis as the dermal
tissue system of the plant
Vascular Tissues
• Secondary xylem is dead at maturity and transports water
essentially through a hollow tube - angiosperms have
tracheary cells are called vessels and tend to have flattened
ends, as well as tracheids
• in gymnosperms the tracheary cells are called tracheids
and usually sharply tapered
• eventually the xylem becomes full of sap and is no longer
used for water transport, then it functions in support and
forms wood
• Phloem cells are called sieve tube elements because of the
sieve like plates at the end of the cells - they are alive at
maturity but are crushed as the plant grows in diameter and
must be continually replaced
• Some sieve cells have companion cells which govern
transport of material through the sieve
Xylem –
Vessels
and
Tracheids
Phloem – sieve elements
Plant growth
• Plant material originating from secondary growth
forms the secondary plant body
• Secondary growth appeared in the Devonian about
380 MYA
• Many plants grow continuously throughout their
life and have no final particular size or shape - this
is called indeterminate growth
• Many plant parts such as leaves, flowers and fruit
do have a characteristic final shape and size and
stop growing at that size - that is called
determinate growth
Garlic Roots
Root vegetables
Fibrous
Tap
Varieties of Root Systems
Soil root traces
Lateral growth of a willow root
Back to the shoot system
Plant Embryo
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
Bud Pinching
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