Leaves
Outline Overview Leaf Arrangements and Types Internal Structures of Leaves Stomata Mesophyll and Veins Specialized Leaves Autumnal Changes in Color Abscission Relevance of Leaves
Overview Some of the food you eat is made of leaves. When you have a salad, you eat the blades of the lettuce leaves. If you have onions in your salad, you re eating swollen leaves. And if you eat celery, you ve eaten giant petioles.
Overview All leaves begin as leaf primordia, tiny embryo leaves in the buds of plants. At maturity, most leaves have a stalk (petiole) and a flattened blade (lamina) with a network of veins (vascular bundles). Some leaves have paired appendages, called stipules, at the base of the petiole. Leaves of flowering plants are associated with leaf gaps and have an axillary bud at the base. May be simple (single blade) or compound (divided into leaflets).
Overview Simple Leaf Compound Leaf
Overview
Overview Pinnately compound leaves have many small leaflets that attach along an axis called a rachis. Palmately compound leaves have several leaflets that attach at the same point at the end of the petiole so that the leaflets look like fingers on a hand. Pinnately compound leaves may be further subdivided into more leaflets and are referred to as bipinnately compound.
Overview Green leaves are designed to capture as much sunlight as possible for photosynthesis, while at the same time protecting the plant s water supply. Lower surfaces of leaves are dotted with stomata which allow carbon dioxide to enter and oxygen and water to diffuse out. Guard Cells control stomatal opening. Transpiration occurs when water evaporates from the leaf surface. Guttation - Root pressure forces water out hydathodes.
Leaf Arrangements and Types Leaves are attached to stems at nodes, with stem regions between nodes known as internodes. Phylotaxy (leaf arrangement) generally occurs in one of three ways: - Alternate - Opposite - Whorled
Leaf Arrangements and Types
Leaf Arrangements and Types Arrangement of veins in a leaf or leaflet blade may also be pinnate or palmate. Pinnately veined leaves have a main midvein within a midrib. - Secondary veins branch from midvein. Palmately veined leaves have several primary veins that fan out from the base of the blade. - Parallel in monocots - Divergent in dicots (reticulate venation)
Internal Structure of Leaves The epidermis is the outer protective layer of cells covering the entire surface of the leaf. Leaves need to exchange gas with the environment for example, they need to take in carbon dioxide for photosynthesis but they also need to minimize water loss. Two features of leaves help them exchange gases without losing too much water. Upper epidermal cells have no chloroplasts. Waxy cuticle often present. Different glands may also be present in the epidermis.
Internal Structure of Leaves A waxy cuticle coats the out surface of the epidermis. The cuticle helps prevent water loss from the leaf to the air. Guard cells within the epidermis control whether the pores in the leaf, called stoma, are open or closed. The lower epidermis of most plants is perforated by numerous stomata. Guard cells originate from the same parent cell, and contain chloroplasts.
Stomata Primary function includes regulating gas exchange between leaf interior and the atmosphere, and the evaporation of water. Cell water pressure regulates guard cells which in turn regulate stomata. By controlling the guard cells, plants make sure that they open their stoma only when it s safe and they won t lose too much water. When the guard cells fill up with water, they swell and pull away from each other, opening the stoma. When the guard cells lose water, they become flaccid and move toward each other, closing the stoma.
Mesophyll and Veins Parenchyma cells make up most of the interior of the leaf. These cells form the main photosynthetic region of the plant, called mesophyll, which forms the majority of the tissue between the upper and lower surfaces of the leaf. Mesophyll may contain two different types of parenchyma cells: The upper part of the mesophyll consists of tall, columnar cells called palisade parenchyma. These cells are packed together directly under the upper epidermis to absorb as much light as possible in their many chloroplasts.
Mesophyll and Veins
Mesophyll and Veins The lower part of the mesophyll contains loosely arranged parenchyma cells, called spongy parenchyma, that have lots of air spaces between them.
Mesophyll and Veins Vascular tissue makes up the veins of the leaf. The vascular tissue brings sugar from the leaves to the stem of the plant [phloem] and also conducts water from the plant out into the leaves [xylem]. Each vein contains xylem and phloem tissue surrounded by a layer of protective, thickwalled fiber cells called the bundle sheath. Veins (Vascular bundles) are scattered throughout the mesophyll.
Mesophyll and Veins The pattern of veins is typically different in monocots and dicots. Monocot leaves usually have parallel veins, where the veins line up in parallel lines. Dicot leaves usually have reticulate, or netted, veins, where the veins form a branching network in the leaf.