UCSC Biology 20B
LEAVES
FALL 2004
Function
• Leaves are the principal photosynthetic organs of the plant.
• Leaves are supported by the stems, which place the leaves in favorable positions for
exposure to light.
• Most of the plant's loss of water vapor occurs through the leaves.
Outer Structure
• The epidermis covers the leaf. The epidermis protects the leaf against physical damage
and pathogens.
• The waxy cuticle of the epidermis is a barrier to the loss of water from the plant.
• The epidermis contains stomata. Stomata are tiny pores that allow gas exchange
between the surrounding air and the photosynthetic cells inside the leaf.
• Stomata are also the main location of water loss from the plant through the process of
transpiration.
• Two guard cells surround each stoma (plural=stomata); these regulate the opening and
closing of the stomata.
• Stomata are randomly arranged in the leaf surface of dicots; stomata are arranged in
rows parallel with the long axis of the leaf in monocots.
• The stomata tend to be more numerous on the bottom surface of a leaf than on top.
This adaptation minimizes water-loss, since water loss occurs more rapidly through
stomata on the sunny upper side of a leaf.
• The pattern of venation is a characteristic difference between monocots and dicots:
o Most dicots have a branching pattern termed netted venation. The veins
branch from one midrib (main vein) of the leaf.
o Monocots have parallel venation. Veins extend parallel to the long axis of the
leaf.
Inner Structure
• The tissue inside the leaf between the upper and lower epidermis is called the
mesophyll (mesophyll literally means "middle of the leaf").
• The mesophyll is divided into two regions:
o The upper half of the leaf contains the palisade parenchyma. Palisade
parenchyma is made of columnar shaped cells.
o The lower half of the leaf is the spongy mesophyll. The spongy mesophyll
contains irregularly shaped cells in large, intricate air spaces.
These air spaces contain CO2 and O2. Both CO2 and O2 are exchanged
through the stomata.
• The mesophyll of the leaf is thoroughly permeated by numerous vascular bundles, or
veins. Veins are continuous with the vascular system of the stem:
o Veins form a complex system bringing xylem and phloem into close contact
with the photosynthetic tissue.
Photosynthetic tissue uses water and minerals from the xylem.
Veins load sugars and other organic products into the phloem for
transport to other parts of the plant.
o The veins of the leaf also serve a support function for the mesophyll of the leaf.
UCSC Biology 20B
LEAVES
FALL 2004
Leaf Adaptations:
• Variations in the structure of angiosperm leaves are to a great extent related to the
availability of resources, habitat, mode of life, or interactions with other organisms.
o FOR EXAMPLE: The availability of water is an important factor affecting
their form and structure. Plants are commonly characterized on the basis of
their water requirements or adaptations.
• Xerophytes are plants with special adaptations to dry environments
• Hydrophytes are plants with special adaptations to wet environments.
o FOR EXAMPLE: Epiphytes grow on the surface of other plants and not in soil.
Many epiphytes, such as bromeliads possess trichomes (epidermal hairs) for
the absorption of water and minerals.
o FOR EXAMPLE: Trichomes are also thought to provide a physical and
chemical defense against insects.
• Trichomes prevent insect mouth-parts from reaching the surface of the
leaf (physical barrier).
• Glandular hairs, which have a secretory function, may also provide a
chemical defense against other organisms.
• Example: several species belonging to the genus Pelargonium.
Xerophyte Adaptations
Leaves are modified in various ways to reduce the rate of transpiration:
• These plants often have small, thick leaves to reduce the surface area relative to
volume. This limits water loss.
o Examples of plants with fleshy leaves: Jade plant and species belonging to the
Sedum genus, Peperomia genus, and Crassula genus.
• Plants may possess hairs (trichomes) on the epidermis to protect from drying winds, to
dissipate heat, and avoid overexposure to light. Epidermal hairs lower leaf
temperatures and decreases water loss.
o Example: “old man cactus” (species belonging to the genus Espostoa)
• Plants often form a heavy layer of cuticle over the leaf epidermis to retard water loss.
o Example: “wax plant” (species belonging to the genus Hoya).
• Some plants have several epidermal layers (multiple epidermis) that serve as a waterstorage tissue.
o Example: “rubber plant” (the species Ficus elastica)
• Cacti have spines (which are actually modified leaves) rather than typical leaves.
o The spines may serve to reflect light and/or to dissipate heat.
o Photosynthesis is confined to the fleshy stems.
• The blue, gray or glaucous tinge (from the waxy covering) of some plants reflects light
as well as minimizes water loss.
o Example: “ghost plant” (species belonging to the genus Graptopetalum)
• Fenestration. Some succulents bear leaves with window-like, transparent areas in their
tips to admit light. In the wild, the plant is embedded in sand with only the leaf tips
exposed to the light.
o Example: “window plant” (species belonging to the genus Fenestraria)
• Some species have stomata in sunken cavities below the lower leaf epidermis. These
cavities are also called stomatal crypts. The hairs covering these crypts presumably
trap moist air.
UCSC Biology 20B
LEAVES
FALL 2004
Hydrophyte Adaptations
Leaves on aquatic plants that are floating or submersed have some interesting anatomical
adaptations:
• Floating leaves are exposed to two markedly different environments at the same time.
Their special adaptations include the following.
o Almost all have evolved toward a circular shape. This shape provides
maximum protection against tearing when leaves are subjected to wind and
wave action, since leaves with indentations or lobbing would provide points
where stresses and tears would be more likely to develop.
o The upper surface is usually covered with waxy cuticle. A waxy cuticle sheds
water and helps to prevent immersion.
o Stomata are located on the upper epidermis so that gas exchange can occur.
o Leaf tissue consists of large air spaces in the mesophyll. These air spaces
maintain buoyancy in the floating leaf and also permit the diffusion of oxygen.
Example: “water lily” (species belonging to the genus Nymphaea)
•
Submersed leaves
o Most submersed leaves have a very high surface-to-volume ratio.
The large surface area permits greater exchange of gases between the
plant tissue and the water.
Leaves of this type tend to be ribbon-like or strap-shaped.
• Example: species belonging to the genus Elodea
o The surface of submersed leaves:
Is NOT covered with a waxy cuticle.
Does NOT have epidermal hairs. A “hairless” epidermis allows the
diffusion of dissolved gases in the water.
TERMS TO KNOW
Epidermis
Upper and lower epidermis
Cuticle
Vein
Stomata
Venation
Guard cells
Stomatal crypts
Palisade parenchyma
Stoma
Spongy mesophyll
Xerophyte
Vascular bundles
Hydrophyte
Xylem
Epiphyte
Phloem
Chemical defense
Stomatal crypt
Genus
Trichomes
Species