Leaves

What is a leaf?

A plant organ that performs the function of photosynthesis, or creating sugars from carbon dioxide and water.
They are usually flattened and wide, appearing two-dimensional, to maximize surface area for light interception and gas exchange.
- N.B. some stems can also photosynthesize, and evolution has even created leaf-like stems, called cladophylls. These stems would still have axillary buds, not found in leaves, and the anatomy of leaves and stems differ.
Leaves appear later in geologic history than stems or roots, and have independently evolved at least 6 times.

Vascular tissues move water and minerals from roots through stems to the leaves
The leaves absorb carbon dioxide through stomata, and in the presence of light, combine these materials to make sugars
The leaves shuttle the sugars created in photosynthesis to areas of active growth

Simple leaves: leaf with a single blade (e.g. maple leaf, oak leaf, orchid leaf)
Compound leaves: a single leaf that appears to be many small leaves (e.g. palm leaf, fern frond, cycad leaf)

Above: Simple leaf (left) and a compound leaf (right)

Coleus Stem Tip (l.s.)
- LABEL: Apical meristem, Procambium, Ground meristem, Protoderm, Leaf primordium, Bud primordium, Young leaf
Monocot leaf (x.s.)
- LABEL: Epidermis, Vascular bundles, Bundle sheath cells, Bulliform cells, Mesophyll, Stomata
Dicot leaf (x.s.)
- LABEL: Epidermis, Vascular bundles, Palisade mesophyll, Spongy mesophyll, Stomata

Above: Longitudinal section of the stem tip of Coleus (x40)

Above: Cross-section of a monocot leaf (x40)

Above: Cross-section of a dicot leaf (x40)

How are leaves different from stems?
What is the structure of a simple or compound leaf?
How do leaves develop?
How do the terms chlorenchyma and aerenchyma pertain to leaves, especially eudicots?
How does internal and external leaf structure differ in monocots and eudicots?
How are the leaves, stems, and roots interconnected to facilitate evapo-transpiration with the least amount of energy expended by the plant?
What is developmental plasticity, and how does this manifest itself in leaves?
What are bracts, and what is there function in plants?
What are leaf adaptations for arid environments?
What is the difference between a spine, thorn, and prickle?
In what manner are both autumn leaf changes and deciduousness, considered to be adaptations?
How do plants prepare for and promote leaf loss in the autumn, and where do new leaves appear in the spring?
What is the difference between a bulb and a corm?
How are bulbs and corms examples of plant structures with specialized roots, stems, and leaves?
How are reproductive leaves developmentally different?
What is the evolutionary function of carnivorous leaves?
See also the flower section for comparison questions

Plants are Cool, Too!: Pitcher Plants
Time lapse Tendrils
Why Is Red Fall Color Nearly Absent in Northern Europe but Prevalent in North America? (Appalachian State Univ)
Origin of Leaves: Currently, researchers indicate that leaves have evolved several times in different lineages. For more information, check out the Plant Evolution page.