Biology 1A Nov 3, 1998
Lecture 18 - Leaves
Page Contents :
Overview of Lecture:
The origin of leaves.
Leaf functions and specializations.
Overview of Lab:
(A bullet indicates a handout.)
I. The origin of leaves.
A. The developmental origin of leaves
II. Leaf Morphology.
1. leaf primordia
B. The evolutionary origin of leaves
2. marginal meristems
1. Earliest vascular plants had no leaves
2. Leaves have evolved at least twice -- microphylls and megaphylls
3. Microphyll origins
a. small projections formed; enations
4. Megaphyll origins
b. later, single vascular strand grew toward and into the enation
c. result is a microphyll, with single unbranched vein
d. found only in one group of plants (Lycophyta)
a. ancestors had dichotomous branching
b. overtopping, planation, webbing
c. Telome theory
d. ferns & all seed plants
A. phyllotaxy - the arrangement of leaves on the stem
III. Leaf Anatomy.
1. alternate - one leaf per node
B. Parts of the leaf
2. opposite - two leaves at same node
3. whorled - three or more leaves at same node
1. Three basic parts of the leaf: blade, petiole, stipules
C. Leaf shape
2. One or more of these parts may be absent in a leaf
1. dissection of the blade
D. venation - pattern of the vascular system in the leaf
a. simple leaf - one interconnected blade
2. margin of the blade -- entire, lobed, toothed
b. compound - several separate blades
i. pinnate - like a feather; ash walnut, hickory
ii. palmate - like a fan; buckeye
iii. doubly compound - ferns, mimosa
1. midvein present -- pinnate, palmate, netted
2. no midvein -- parallel, dichotomous (Ginkgo)
A. Overview of tissue system locations
IV. Leaf functions and specializations.
1. abaxial & adaxial
2. stomata, flanked by guard cells
4. specialized epidermal cells
a. buliform cells
b. trichomes, glands
1. mesophyll - "middle of the leaf"
D. Vascular bundles (veins)
2. palisade mesophyll
a. located on adaxial side
3. spongy mesophyll
b. may contain more than 80% of the leaf's plastids
c. controls light intensity and damage by reducing light passing through
a. spongy appearance because of air spaces, allowing free gas flow
b. primary site of photosynthesis in vascular plants
1. often enclosed by bundle sheaths of sclerenchyma fibers
2. xylem on adaxial, phloem on abaxial. Why?
A. Leaves are the primary photosynthetic organ of vascular plants
B. Sun vs shade leaves
1. sun leaves - smaller, thicker, more mesophyll
C. Specializations for extreme environments
2. shade leaves - larger, thinner, fewer mesophyll layers
D. Other leaf specializations
2. carnivory - leaf is modified to trap insects for trace nutrients
3. hydrophytes (aquatic plants)
4. xerophytes (desert plants)
1. tendrils - elongated leaves for climbing and attaching
2. spines - sharp stiff leaves for defense
3. bracts - floral leaves; often colorful to attract pollinators
abaxial entire palmate
abscission guard cell petiole
adaxial hydrophyte phyllotaxy
apex internode pinnate
blade leaf primordium planation
bracts lobed simple
buliform cells margin spine
bundle sheath marginal meristem spongy mesophyll
carnivory megaphyll stipules
compound leaf mesophyll Telome Theory
cuticle microphyll tendril
deciduous midvein venation
dichotomous node webbing
dissected blade overtopping whorled
enation palisade mesophyll xerophyte
- Describe the process by which a leaf develops from the shoot apical meristem.
- How did enations evolve into microphylls? In which group of plants are microphylls found?
- How did megaphylls evolve? How is this different from the origin of microphylls?
- Define "phyllotaxy". How do alternate, opposite, and whorled phyllotaxies differ from each other?
- What are the three basic parts of a leaf? Which part may be dissected?
- Draw a simple leaf, a pinnately compound leaf, and a palmately compound leaf. Explain the structural differences among them.
- What kind of venation do monocot leaves have? What kinds can dicot leaves have? Which plant has dichotomous venation?
- Diagram a typical leaf cross section, indicating the location and arrangement of the three tissue systems.
- How is leaf epidermis like stem epidermis? How is it different?
- What environmental factors may cause stomata to close? to open up?
- Which epidermal cells have chloroplasts? Which may lack cuticle? Which make lemons smell nice?
- What are buliform cells? How do they function? In which plants are they found?
- Name and describe the two mesophyll tissues in a typical dicot leaf. Which has more chloroplasts? Which does more photosynthesis? Which is often absent in monocots?
- Which vascular tissue is located on the adaxial side of a leaf? Why?
- Compare the vascular arrangement in roots, stems, and leaves. How are they similar? How are they different?
- How do sun and shade leaves on a plant differ? Why?
- What is an abscission layer? What does it do? How does it do this?
- What leaf modifications might you find in a hydrophyte? in a xerophyte?
- What are bracts and what function can they serve?
- What is the difference between anatomy and morphology?
© 1998 Brian R. Speer. These pages for the personal use of students and teachers; any commercial use or publication is strictly prohibited.