Transcript Botany Basics 3

Botany Basics 3
Crop Science 1
Fall 2004
September 28, 2004
Leaf Function
Principal function
 Photosynthesis
 to absorb sunlight to manufacture plant
sugars
 Surfaces flattened to present a large area
for efficient light absorption
Leaf Structure
 Blade
 expanded thin structure
on either side of the
midrib and usually the
largest, most
conspicuous part of a
leaf
Leaf Structure
 Petiole
 Stem-like appendage
which holds the leaf
away from the stem
 The base is attached
to the stem at a node
 Vary in length or may
be lacking entirely, in
which case the leaf
blade is described as
sessile or stalkless
Leaf Structure
 Leaf axil
 The node where a petiole
meets a stem
 Contain axillary buds
 single buds or bud clusters
 Either active or dormant
 Will develop into stems or
leaves under the right
conditions
Leaf Blade Structure
 A leaf
blade is
composed
of several
layers
Epidermis
 Layer of thick, tough cells on the top and
bottom
 Primary function is to protect the other
layers of leaf tissue
 The arrangement of epidermal cells
determines the leaf's surface texture
 Some leaves have hairs (pubescence),
which are extensions of epidermal cells
Cuticle
 Part of the epidermis
 Produces a waxy layer called cutin
 Protects the leaf from dehydration and disease
 The amount of cutin on a leaf increases with
increasing light intensity
 Move plants from shade into full sunlight gradually
to allow the cutin layer to build up and protect the
leaves from rapid water loss or sunscald
 Repels water
 Many pesticides contain a spray additive to help the
product adhere to, or penetrate, the cutin layer
(surfactant)
Guard Cells
 Special epidermal cells
 Open and close in response to environmental
stimuli (changes in weather and light)
 Conditions that cause a loss of water (high
temperature, low humidity), or the absence of
light stimulate guard cells to close
 Regulate the passage of water, oxygen, and
carbon dioxide into and out of the leaf through
tiny openings called stomata
 In most species, the majority of the stomata are
located on the underside of leaves
Mesophyll
 Located between the upper and lower
epidermis
 Divided into:
 dense upper layer (palisade mesophyll)
 lower layer that contains lots of air space
(spongy mesophyll)
 Located within the mesophyll cells are
chloroplasts
 Where photosynthesis takes place
Types of Leaves
There are many kinds of plant leaves:
 Foliage
 Scale leaves
 Seed leaves
 Spines and tendrils
Modified Leaves
 Storage leaves
 Bracts
Foliage
 Most common and conspicuous leaves
 Primary location of photosynthesis
Scale leaves
 Cataphylls
 Found on rhizomes and buds
 Enclose and protect rhizomes and buds
Seed leaves
 Cotyledons
 Found on embryonic plants
 Store food for the developing seedling
Spines and Tendrils
 Found on plants such as barberry and
pea plants
 Protect a plant
 Help support stems
Storage Leaves
 Found on bulbous plants and succulents
 Store food
Bracts
 Often are brightly colored
 The showy structures on dogwoods and
poinsettias are bracts, not petals
Venation
 The vascular bundles of xylem and
phloem extend from the stem, through
the petiole, and into the leaf blade as
veins.
 The term venation refers to how veins
are distributed in the blade.
 There are two principal types of venation:
parallel-veined and net-veined
Venation
Parallel-Veined Leaves
 Numerous veins run essentially parallel to each
other and are connected laterally by minute,
straight veinlets
 Occur most often on monocotyledonous plants
 The most common type of parallel veining is
found in plants of the grass family, whose veins
run from the leaf's base to its apex
 Another type of parallel venation is found in
plants such as banana whose veins run
laterally from the midrib
Net-Veined Leaves
 Also called reticulate-veined
 Veins branch from the main rib or ribs
and subdivide into finer veinlets then
unite in a complicated network
 More resistant to tearing
 Occur on dicotyledonous plants
Pinnate vs. Palmate
Either
 Pinnate
 The veins extend laterally from the midrib to
the edge
 Apples, cherries, and peaches
 Palmate
 The principal veins extend outward, like the
ribs of a fan, from the base of the leaf blade
 Grapes and maples
Common Blade Shapes
 Lanceolate
 Longer than wide and
tapering toward the
apex and base
 Linear
 Narrow, several times
longer than wide, and
of approximately the
same width
throughout
Common Blade Shapes
 Cordate
 Heart-shaped
 Broadly ovate,
tapering to an acute
apex, with the base
turning in and forming
a notch where the
petiole is attached
Common Blade Shapes
 Elliptical
 About two or three
times as long as wide,
tapering to an acute
or rounded apex and
base
 Ovate
 Egg-shaped, basal
portion wide, tapering
toward the apex
Common Margin Forms
 Entire
 Smooth edge with no
teeth or notches
 Crenate
 Rounded teeth
 Dentate
 Teeth ending in an
acute angle pointing
outward
Common Margin Forms
 Serrate
 Small, sharp teeth
pointing toward the
apex
 Incised
 Sharp, deep, irregular
teeth or incisions
 Lobed
 Incisions that extend
less than halfway to
the midrib
Common Apex Shapes
 Acute
 Ending in an acute
angle, with a sharp,
but not acuminate,
point
 Acuminate
 Tapering to a long,
narrow point
 Obtuse
 Tapering to a rounded
edge
Common Base Shapes
 Cuneate
 Wedge-shaped;
triangular with the
narrow end at the
point of attachment
 Obtuse
 Tapering to a rounded
edge
 Cordate
 Turning in and
forming a notch
Plant Identification
Leaves are useful for plant identification:
 Leaf's venation
 Blade and margin shapes
 Apex and base shape
 Leaf type
 Leaf arrangement
Leaf Type
 Simple leaves
 Leaf blade is a single,
continuous unit
 Compound leaves
 Composed of several
separate leaflets arising
from the same petiole
 Some leaves are doubly
compound
 Leaf type can be
confusing, because a
deeply lobed simple leaf
may look like a
compound leaf
Leaf Arrangement
 Opposite
 Positioned across the
stem from each other,
with two leaves at
each node
 Alternate (spiral)
 Leaves are arranged
in alternate steps
along the stem, with
only one leaf at each
node
Leaf Arrangement
 Whorled
 Arranged in circles
along the stem
 Rosulate
 Arranged in a rosette
around a stem with
extremely short nodes
Leaves as Food
 Leaf blade
 Chives, collards, dandelions, endives, kale,
leaf lettuce, mustard, parsley, spinach and
Swiss chard
 Cluster of fleshy leaf bases
 Leeks, onions, and Florence fennel
 Petiole
 Celery and rhubarb