Chapter 7: Leaves
Morphology and Anatomy of Other Leaf Types
External Structure of Foliage Leaves
Internal Structure of Foliage Leaves
Initiation and Development of Leaves
Flat light-harvesting portion: Leaf Blade/Lamina. The blade's lower side is the dorsal surface, and the larger veins protrude. (abaxial side) The upper side is the ventral surface (adaxial) and is usually smooth. The petiole (stalk) holds the blade out into the light. If there is no petiole, the leaf is called a sessile leaf instead of a petiolate. In some plants the leaf base wraps around the stem to form a sheathing leaf base.
Leaf blades can be simple or compound. A simple leaf has just one part, and compound leaves have a blade divided into several parts. Compound leaves have many small blades called leaflets, each attached by a petiolule to an extension of the petiole, the rachis.
Within leaves are veins, which are bundles of vascular tissue. In eudicot plants they occur in a netted pattern called reticulate venation. At the leaf base (usually in the petiole) is an abscission zone, which is involved in cutting off the leaf when it's life is over.
Water loss through the epidermis is called transpiration, if the soil is too dry the roots cannot replace lost water.
Mesophyll: Collective ground tissues interior to the leaf epidermis. On the upper surface of most leaves is the palisade parenchyma, and this is the main photosynthetic tissue of most plants. In the lower portion of the leaf is the spongy mesophyll, which is arenchyma that allows CO2 to diffuse rapidly from the stomata into all other parts of the leaf.
Vascular Tissues: Leaves usually have midribs, or midveins, that lateral veins emerge from and branch into minor veins. They may also have a sheath of fibers, called a bundle sheath around the vascular tissues. Larger veins often have a mass of fibers above and below or just above and below called the bundle sheath extension.
Petiole: Considered part of the leaf and are the transition between the stem and the lamina. One, three five ore more vascular bundles called leaf traces branch from stem vascular bundles and diverge towards the petiole. In many plants, the petiole bears two small flaps of tissue at the base called stipules. These serve various functions.
Basal Angiosperms and Eudicots: Just interior to the protoderm growing outward is a protrusion called the leaf primordium. It extends upward as a narrow cone and grows so rapidly that it becomes taller than the shoot apical meristem.
Monocots: The primordium becomes a cylinder that completely or almost completely encircles the shoot apical meristem. It grows upwards as a sheathing leaf base, and the original leaf primordium gives rise to the lamina.
Succulent Leaves: Advantageous for water retention. Reduces capacity for carbon dioxide uptake. Lack of air spaces in the mesophyll makes it more transparent, allowing light to penetrate further into the leaf.
Sclerophyllous Foliage Leaves: Limited sclerenchyma in the leaves makes them soft, flexible, and edible. Usually have a very thick and abundantly waxy cuticle.
Leaves of Conifers: Needles either short or long, mostly perennial. Needles last for about 5 years, and their vascular bundles can produce new phloem each year but no new xylem are produced.
Bud Scales: Dormant shoot apical meristems are protected from low temps and the drying action of wind during winter by bud scales, forming a tight layer around the stem tip. Bud scale structure is different from foliage leaves. They are small and rarely compound, so wind damage is not a risk. Petioles are short or absent, and they must be tougher and waxier than regular leaves. Bud scales frequently produce a layer of corky bark on the exposed portions, providing greater protection.
Spines: Cacti have 2 types of leaves, the green body has microscopic green leaves, and the spines are their axillary buds. The spines are modified leaves of axillary buds. Mutations occurred that inhibit lamina formation, being selectively advantageous. No mesophyll parenchyma or vascular tissue is present, instead consisting of closely packed fibers. After these fibers mature, they deposit lignin in their walls, which makes them hard and resistant to decay. They then die and dry out, hardening more.
Tendrils: Are a form of modified leaf, growing indefinitely and containing cells that are capable of sensing contact with an object. When it touches something, the side facing the object stops growing but the other side continues to elongate, causing it to wrap around the object. No lamina forms.
Leaves with Kranz Anatomy: A distinct type of leaf anatomy, lacking palisade parenchyma and spongy mesophyll but have prominent bundle sheathes composed of large chlorophyllous cells. Surrounding each sheath is a ring of mesophyll cells that appear to radiate from the vascular bundle. They have a mechanism of carbon dioxide transport that requires this special anatomy.
Insect Traps: Active traps can move, passive traps cannot. The lamina is tubular rather than flat, and it secretes a watery digestive fluid. The "throat" of the pitcher contains numerous trichomes that point toward the liquid. Some have upper surfaces covered in glandular trichomes that secrete a sticky liquid, and after an insect is caught the other adjacent trichomes are stimulated to bend toward the victim. Venus flytraps has turgid and swollen motor cells along the upper side of the midrib. If an insect brushes two of the trigger hairs, the midrib motor cells lose water quickly, and the trap rapidly closes. After the absorption is complete, the motor cells fill with water, swell, and force the trap open.