Plant Structures: Leaves
- Internal features
- External features
- Leaf arrangement on stem
- Leaflet arrangement on petiole
- Leaf shape
- Leaf margin
- Leaf types (leaf venation)
- Modified leaves
Explain the science behind the following gardening questions:
1. Last spring my tulips were magnificent. As blooms faded, I removed the blossoms and foliage so it would not detract from other spring flowers coming into bloom. This year, most of the tulips did not come back. Why?
Leaves are the principle structure, produced on stems, where photosynthesis takes place. Cacti are an exception. The leaves are reduced to spines, and the thick green, fleshy stems are where photosynthesis takes place.
- To compete for light for photosynthesis, i.e., the manufacture of sugars
- Evapotranspiration from the leaves is what moves water and nutrients up from the roots.
- Small openings on the leaf, known as stomata,regulate moisture and gas exchange (water and carbon dioxide) and temperature (cooling effect as water vapor escapes through stomata).
- Horticultural uses
- Aesthetic qualities
- Feed and food
- Mulch and compost
- Plant identification
- Propagation from cuttings
- Summer cooling (Evaporative cooling accounts for 70-80% of the shading impact of a tree.)
- Wildlife habitat
- Wind, dust and noise reduction
The leaf blade is composed of several layers as follows: [Figure 1]
- Epidermis – Outer layer of tissues
- Cuticle – Waxy protective outer layer of epidermis that prevents water loss on leaves, green stems, and fruits. The amount of cutin or wax increases with light intensity.
- Leaf hairs – part of the epidermis
- Palisade layer – A tightly packed layer of parenchyma tissues filled with chloroplasts for photosynthesis.
- Chloroplasts – Sub-cellular, photosynthetic structures in leaves and other green tissues. Chloroplasts contain chlorophyll, a green plant pigment that captures the energy in light and begins the transformation of that energy into sugars.
- Vascular bundle – Xylem and phloem tissues, commonly known as leaf veins.
- Spongy mesophyll – Layer of parenchyma tissues loosely arranged to facilitate movement of oxygen, carbon dioxide, and water vapor. It also may contain some chloroplasts.
- Stomata – Natural openings in leaves and herbaceous stems that allow for gas exchange (water vapor, carbon dioxide, and oxygen).
- Guard cells – Specialized kidney-shaped cells that open and close the stomata.
Figure 1. Leaf cross section with stomata.
- Leaf blade – Flattened part of the leaf
- Petiole – Leaf stalk
- Stipules – Leaf-like appendages at the base of the leaf.
Figure 2. Leaf external features.
For plant identification purposes, the shape of the leaf margin, leaf tip and leaf base are key features to note. Remember, a leaf begins at the lateral or auxiliary bud.
- Alternate – Arranged in staggered fashion along stem (willow)
- Opposite – Pair of leaves arranged across from each other on stem (maple)
- Whorled – Arranged in a ring (catalpa)
- Rosette – Spiral cluster of leaves arranged at the base (or crown) (dandelion)
Figure 3. Leaf arrangement on stem.
- Simple – Leaf blade is one continuous unit (cherry, maple, and elm)
- Compound – Several leaflets arise from the same petiole
- Pinnately compound – Leaflets arranged on both sides of a common rachis (leaf stalk), like a feather (mountain ash)
- Palmately compound – Leaflets radiate from one central point (Ohio buckeye and horse chestnut)
- Double pinnately compound – Double set of compound leaflets
Figure 4. Leaf arrangement on petiole.
Note: Sometimes identifying a "leaf" or "leaflet" can be confusing. Look at the petiole attachment. A leaf petiole attaches to the stem at a bud node. There is no bud node where leaflets attach to the petiole.
Leaf shape is a primary tool in plant identification. Descriptions often go into minute detail about general leaf shape, and the shape of the leaf apex and base. Figure 5 illustrates common shapes as used in the Manual of Woody Landscape Plants.
Figure 5. Leaf shapes.
Shape of the leaf apex (tip) and base is another tool in plant identification. Figures 6 and 7 illustrate common tip and base styles as used in the Manual of Woody Landscape Plants.
Figure 6. Leaf tip shapes.
Figure 7. Leaf base shapes.
The leaf margin is another tool in plant identification. Figure 8 illustrates common margin types as used in the Manual of Woody Landscape Plants.
Figure 8. Leaf margin shapes.
- Scale-like – Mature leaves common on most junipers and arborvitae
- Awl-shaped – Juvenile leaves common on some junipers
- Linear-shaped – Narrow flat needles of spruce, fir, and yews
- Needle-like – In pine, the single, bundle, or cluster of needles makes a rounded shape
Figure 9. Example of foliage types on Conifers
Dichotomous venation – Somewhat parallel vein sections, forming a 'Y', found in Ginkgo trees.
Figure 10. Ginkgo leaf with dichotomous venation.
Parallel venation – Veins run in parallel lines. (monocot plants, e.g. grasses, lilies, tulips).
Figure 11. Monocot leaf with parallel venation.
- Net-veined or reticulate-veined – Leaves with veins that branch from the main rib and then subdivide into finer veinlets. (dicot plants)
- Pinnate venation – Veins extend from a midrib to the edge (elm, peach, apple, cherry).
- Palmate venation – Veins radiate fan-shaped from the petiole (maple, grapes).
Figure 12. Leaf venation.
- Adhesive disc – Modified leaf used for attachment mechanism. Sometimes referred to as a holdfast. (Boston ivy)
- Bract – Specialized, often highly colored leaf below flower that often serves to lure pollinators. (Poinsettia, dogwood)
- Tendril – Modified sinuous leaf used for climbing or as an attachment mechanism. (Virginia creeper, peas, grapes)
- Thorn – Modified leaf. (barberry, pyracantha)
Figure 13. Thorns on a stem are an example of leaf modification.
CMG GardensNotes on vegetable gardening
- Horticulture Classification, #121
- Taxonomic Classification, #122
- Plant Structures: Cells, Tissues, and Structures, #131
- Plant Structures: Roots, #132
- Plant Structures: Stems, #133
- Plant Structures: Leaves, #134
- Plant Structures: Flowers, #135
- Plant Structures: Fruit, #136
- Plant Structures: Seeds, #137
- Plant Growth Factors: Photosynthesis, Respiration and Transpiration, #141
- Plant Growth Factors: Light, #142
- Plant Growth Factors: Temperature, #143
- Plant Growth Factors: Water, #144
- Plant Growth Factors: Hormones, #145
- Homework: How Plants Grow. #147
- References and Review Questions: How Plants Grow, #120
- Worksheet: Plant Structures, #146
- CMG GardenNotes are available online at www.cmg.colostate.edu
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Revised September 2014