LEAVES
Leaf Arrangement and Types
- The variety of shapes, sizes, and textures of leaves seems to be almost infinite. The leaves of some of the smaller duckweeds are less than 1 millimeter (0.04 inch) wide.
- Different forms of leaves, like scales, bracts, and spines, can be produced on the same plant along with photosynthetic leaves.
- Leaves can be tubular, feathery, cup shaped, needlelike, and can have various textures like smooth, hairy, waxy, or sticky.
Internal Structure of Leaves
- A typical leaf has three main regions when cut transversely: epidermis, mesophyll, and veins (vascular bundles).
- The epidermis is a single layer of cells covering the leaf surface, with the lower epidermis containing stomata (tiny pores) for gas exchange.
Stomata
- Stomata are tiny pores found on the lower epidermis of most plants, used for gas exchange and water regulation.
Mesophyll and Veins
- Photosynthesis mainly occurs in the mesophyll between the epidermal layers, with the palisade mesophyll containing most chloroplasts and the spongy mesophyll having abundant air spaces.
- Veins (vascular bundles) within the mesophyll consist of xylem and phloem surrounded by bundle sheath cells, transporting water and nutrients throughout the plant.
Specialized Leaves
- Leaves have evolved to allow plants to thrive in a wide range of ecological niches.
Shade Leaves:
- Leaves in the shade are larger, thinner, and have fewer well defined mesophyll layers and chloroplasts compared to sun exposed leaves.
- They have fewer hairs and adaptations to thrive with less intense sunlight and heat.
Leaves of Arid Regions:
- Develops various leaf adaptations to cope with limited water, extreme temperatures, and high light intensity.
- They may have succulent, water retaining leaves or no leaves at all, relying on stems for photosynthesis.
Leaves of Aquatic Areas
- The submerged leaves of plants that grow in water usually have considerably less xylem than phloem, and the mesophyll, which is not differentiated into palisade and spongy layers, has large air spaces. Other modifications are described in the sections that follow.
Tendrils
- Plants use modified leaves as tendrils for climbing or supporting weak stems.
- Tendrils are sensitive to touch and wrap around objects when stimulated.
Spines, Thorns, and Prickles
- -Desert plants have spines that help minimize water loss and offer protection from animals.
Storage Leaves
- Succulent leaves in desert plants have adaptations for water storage, including large, thin-walled parenchyma cells with large vacuoles.
Flower-Pot Leaves
- Dischidia, a tropical epiphyte, forms leaf pouches for ant colonies.
Window Leaves
- Windows leaves have ice-cream cone shapes with thick, transparent epidermis, few stomata, and waxy cuticles.
Reproductive Leaves
- Walking fern leaves produce new plants at their tips, forming three linked generations.
Floral Leaves (Bracts)
- Bracts are specialized leaves found at the bases of flowers or flower stalks.
Insect-Trapping Leaves
- Nearly 200 flowering plant species have specialized insect-trapping leaves.
Pitcher Plants
- The blades of leaves of many pitcher plants are flattened and function like those of any other leaves.
Sundews
- Sundews are tiny plants with roundish to oval leaves covered in up to 200 glandular hairs with sticky fluid at the tips.
Venus's Flytraps
- Venus's flytrap has leaves shaped like an old-fashioned steel trap found in wet areas of North Carolina and South Carolina.
Bladderworts
- Bladderworts are found submerged in shallow water along the margins of lakes and streams with finely dissected leaves containing tiny bladders.
Autumnal Changes in Leaf Color
- Leaves of many deciduous plants exhibit a variety of colors in autumn before dropping, caused by breakdown of chlorophyll and the revelation of other pigments like carotenes and xanthophylls.
- The breakdown of chlorophyll in fall is influenced by factors like a gradual reduction in day length.
- Water Soluble pigments like anthocyanins and betacyanins accumulate in leaf cells, creating red, blue, or intermediate shades based on acidity levels.
- Varying pigment combinations lead to diverse fall leaf colors, notably in the Eastern Deciduous Forest, North America.
- Fall coloration occurs in temperate zones with deciduous trees and shrubs.
Abscission
- Deciduous plants shed their leaves seasonally through a process called abscission, which also occurs in evergreen trees but gradually.
- Abscission involves changes in an abscission zone near the leaf's base, where hormonal changes lead to the differentiation of protective and separation layers of cells.
- The protective layer becomes coated with fatty suberin, while the separation layer develops gelatinous cells that swell and may divide.
- Environmental changes trigger the breakdown of pectins in the separation layer cells, leading to the leaf's detachment from the stem with the help of wind and rain.
Human and Ecological Relevance of Leaves
Human Uses of Leaves:
● Food - Many leaves are consumed as food, such as cabbage, parsley, lettuce, spinach, and chard. Petioles, like those of celery and rhubarb, are also eaten.
● Spices and Flavorings - Numerous spices and flavorings are derived from leaves, including thyme, marjoram, oregano, tarragon, peppermint, spearmint, wintergreen, basil, dill, sage, cilantro, and savory.
● Dyes - Leaves can be used to extract dyes, such as yellow dye from bearberry, reddish dye from henna, and pale blue dye from blue ash.
● Cordage Fibers - Leaves are a source of cordage fibers for ropes and twines. Various species of Agave are responsible for about 80% of global production.
● Bowstring Fibers - Bowstring fibers are derived from a relative of the common houseplant Sansevieria.
● Manila Hemp Fibers - Manila hemp fibers, used in cordage and textiles, are obtained from the leaves of a close relative of the banana.
● Panama Hats - Panama hats are crafted from the leaves of the panama hat palm.
● Thatching Material - Palms and grasses are used as thatching material for huts and buildings in tropical regions.
● Fuel - In the high mountains of Chile and Peru, the leaves of the yareta plant are used as fuel.
● Oils - Many plant leaves produce oils such as petitgrain oil (from orange tree leaves), lavender, patchouli, lemongrass, citronella, eucalyptus, camphor, cajeput, and pennyroyal. These oils have various uses, including scenting soaps, colognes, perfumes, and mosquito repellents.
● Medicinal Drugs - Leaves are a significant source of drugs used in medicine. Cocaine, derived from plants native to South America, has been used medicinally as a local anesthetic, but its use as a narcotic has become problematic.
● Belladonna - Belladonna, obtained from the leaves of the deadly nightshade, has been used in medicine for centuries. It contains atropine, used in shock treatments, eye dilation, pain relief, and to slow secretions during surgery. Scopolamine, another belladonna derivative, is used in tranquilizers and sleeping aids.
● Digitalis - The leaves of the foxglove are the source of digitalis, a drug used for centuries to regulate blood circulation and heartbeat.
● Tobacco - Tobacco, a widely consumed leaf, is used for smoking, chewing, and snuff.
● Shade - Humans use shade trees in landscaping for cooling and aesthetic purposes.