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Stems, Leaves and Seeds
by Evil Mr. Bleecker
Woody plants in an
evolutionary context
Note that the
evolution of
photosynthesis
occurred LONG
before plants
invaded terrestrial
environments.
“Woodiness”
evolved after plants
invaded land in
response to intense
competition for light.
Dicots: all true “woody” angiosperms are
dicots, but not all dicots are woody species
Saguaro cactus
Hepatica americana
Monocots
Coconut palm
rice
Shoot System
Organization of the plant body
Vegetative organs
are the leaf, stem
and root
The body of a plant
has a root system
and a shoot
system.
Root System
Organization of Stems
Shoot tip
produces new cells
that elongate and
add length to the
stem.
Monocot and dicot traits
Herbaceous (nonwoody) Stems

Mature herbaceous stems exhibit only
primary growth.

The outermost tissue is the epidermis
(not bark), which is covered by waxy
cuticle.
Herbaceous dicot stem
-vascular bundles are in a distinct ring
Monocot stem
-vascular bundles are scattered throughout
Woody Stems



A woody plant has both primary and
secondary tissues.
Primary tissues are new tissues formed
each year from primary meristems right
behind apical meristem.
Secondary tissues develop during the
second and subsequent years of growth
from lateral meristems (vascular cambium
and cork cambium).
Secondary growth, (annual growth)
increases the girth of a plant.
Trees undergo secondary growth because
of a change in vascular cambium.
The secondary tissues produced by the
vascular cambium, are called secondary
xylem and secondary phloem,
Secondary growth in a Dicot
stem
Pith rays are
composed of living
parenchyma cells (a
type of xylem) that
allow materials to
move laterally.
Section of woody stem
Spring wood
followed by
summer
wood makes
up one
year’s
growth or
annual ring.
The bark of a
tree contains
cork, cork
cambium, and
phloem.
A woody stem
has three
distinct areas:
the pith, the
wood, and the
bark.
Annual Rings
This tree had a pith
date of 256 BC and an
outer ring of about AD
1320, making this
tree nearly 1,600
years old when it died
(it's about 3 feet
across)!
(photo © H.D.
Grissino-Mayer and
R.K. Adams).
Tree trunk
Modified stems
Cross section of tree stem
Exercise





1. How old is this tree?
2. How many rainy
seasons occurred during
the life of this tree? In
what years did
they occur?
3. How many dry seasons
occurred during the life of
this tree? In what years
did they occur?
4. During what year of
growth was there a forest
fire?
5. What would happen if a
porcupine completely
chewed away the phloem
tissue around the trunk of
the tree? Why?
Raven et al. Fig. 27-6
Development of primary vascular bundles
Development of a ring of vascular cambium
Early secondary growth
Outer bark
Inner bark
Vascular cambium
Secondary xylem
Organization of primary tissues in a Young Dicot Stem (eg. Bean)
The primary tissues of a Young Monocot Stem (eg. Corn)
Fig. 9.20
Leaf diversity
The leaves of a
cactus are spines
attached to a
succulent stem.
Climbing leaves
are modified
into tendrils.
The leaves of a few
plants are
specialized for
catching insects.
Blade
Petiole
Attaches to the
node here
Monocots:
Parallel veins (sugarcane, corn)
Flowers have 3 or multiples of 3
(6,9,12, etc.) parts
Dicots:
Veins form a net pattern (oak
tree)
Flowers have 4 or 5 or
multiples of 4 or 5 (8,10, etc.)
parts
Dicot leaves
Generalized Leaf Anatomy – Dicot Leaf
Typical Monocot Leaf X-Section
Corn, Sugar Cane
Vein
Epidermis
Phloem
Xylem
Stoma
Leaf Stomata: Allow Gas Exchange
Guard cells with
chloroplasts
Stomata in
Zebrina leaf
epidermis
Stoma
Subsidiary
cells
Figure 1. Guard cell enriched
extract used for cloning and
analysis of phosphoinositide
signaling genes.
Figure 2. HIC promoter trap lines
show guard cell specific GUS
expression. The HIC gene is
involved in controlling stomatal
development in response to
elevated CO2.
Guard Cells
Leaf surfaces are dotted with millions of
stomata such as this one. This stomate is
lined by two guard cells that control its
aperture. Because control requires
movement, and movement requires energy,
these cells contain numerous
mitochondria and chloroplasts (the little
green photosynthetic factories that look a bit
like brussel sprouts in this shot). Thus they
are the only cells in the epidermis that are
green.
Monocot Seeds
Monocot
seeds have:
Endosperm
seed coat
endosperm
cotyledon
coleoptile
One
cotyledon
radicle
Dicot Seeds
Dicot seeds have:
No
endosperm
at maturity
Two
cotyledons
Seed coat
First leaves
Early root
Cotyledon
Development of Seeds
Function of seed structures:
Endosperm
Nutrition for embryo
Cotyledons
Nutrition for embryo
Overall, what is the function of
a seed?
1.
Protects young embryo
2.
Provides nutrition for the
developing embryo
3.
Allows dispersal of the embryo
over time & space
Seed to Seedling