Biology 1A    Nov 3, 1998
Lecture 18 - Leaves

Page Contents :

• Reading Assignment
• Overview of Lecture
• Overview of Lab
• Lecture Outline
• Vocabulary
• Study Questions

Stern: Chapter 7

Overview of Lecture:

The origin of leaves.
Leaf morphology.
Leaf anatomy.
Leaf functions and specializations.

Overview of Lab:

(A bullet indicates a handout.)
• Leaves.

A. The developmental origin of leaves
1. leaf primordia
2. marginal meristems
B. The evolutionary origin of leaves
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
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)
4. Megaphyll origins
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
1. alternate - one leaf per node
2. opposite - two leaves at same node
3. whorled - three or more leaves at same node
B. Parts of the leaf
1. Three basic parts of the leaf: blade, petiole, stipules
2. One or more of these parts may be absent in a leaf
C. Leaf shape
1. dissection of the blade
a. simple leaf - one interconnected blade
b. compound - several separate blades
i. pinnate - like a feather; ash walnut, hickory
ii. palmate - like a fan; buckeye
iii. doubly compound - ferns, mimosa
2. margin of the blade -- entire, lobed, toothed
D. venation - pattern of the vascular system in the leaf
1. midvein present -- pinnate, palmate, netted
2. no midvein -- parallel, dichotomous (Ginkgo)

A. Overview of tissue system locations
B. Epidermis
1. abaxial & adaxial
2. stomata, flanked by guard cells
3. cuticle
4. specialized epidermal cells
a. buliform cells
b. trichomes, glands
C. Mesophyll
1. mesophyll - "middle of the leaf"
2. palisade mesophyll
a. located on adaxial side
b. may contain more than 80% of the leaf's plastids
c. controls light intensity and damage by reducing light passing through
3. spongy mesophyll
a. spongy appearance because of air spaces, allowing free gas flow
b. primary site of photosynthesis in vascular plants
D. Vascular bundles (veins)
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
2. shade leaves - larger, thinner, fewer mesophyll layers
C. Specializations for extreme environments
1. abscission
2. carnivory - leaf is modified to trap insects for trace nutrients
3. hydrophytes (aquatic plants)
4. xerophytes (desert plants)
D. Other leaf specializations
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?