The Form and Structure of Leaves: The Shoot System II

The Functions of Leaves The leaf is organized to maximize sugar production (photosynthesis) while minimizing water loss (transpiration). Main Leaf Functions - Photosynthesis - Transpiration (The main driving force for water transport up the plant). Transpiration also cools the leaf. - Modification (Modified leaves can provide water storage and/or protection for the plant). Leaves are initiated at the Shoot Apical Meristem (SAM ).

Typical dicot (a) and monocot (b) leaves compared. Dicot leaves usually have netted venation and a midrib; monocot leaves usually have parallel veins.

Leaf Types Red buckeye is palmately compound, black locust is pinnately compound, and honey locust is double pinnately (bipinnately) compound.

Tissues of a Typical Leaf Blade

This slide is used to describe the previous slide. The Epidermis covers both sides of the leaf. It is covered by a waxy cuticle made of cutin to minimize water loss. Guard cells (mostly on the bottom side) control the opening of the stomates. Palisade Mesophyll Cells (Parenchyma) are packed tightly to more efficiently absorb sunlight. These cells are at right angles to the epidermis and are often rectangular in shape. Spongy Mesophyll Cells (Parenchyma) are loosely packed to allow for more efficient gas exchange from their cells surfaces. These cells are often more irregular in shape. Vascular Bundles (Veins) are located in the mesophyll are made up of xylem (water and nutrient-carrying) and phloem (sugar- carrying). Bundle Sheath Cells are specialized mesophyll cells (parenchyma) that surround the veins and separate them from the rest of the mesophyll.

Lilac Leaf in Cross Section Showing the Midrib

Leaf Beginnings Primodium – the beginning or origin of any part of an organ. Remember that procambium is a primary meristem and it will give rise to primary vascular tissues. These figures represent the develop of leaf primodia (plural of primodium).

Leaf Form and Specialized Leaves Leaf shape may depend on the plant’s age and the environment. Heterophylly – Different leaf shapes on a single plant. Young bean plants. The plant on the left is younger.

Effect of environmental conditions on the development of buttercup leaves. Water Leaves (thin and deeply lobed) versus Air Leaves (thicker and reduced lobes). On a tree, certain leaves receive more sun (Sun Leaves) and other receive less sun (Shade Leaves). Sun leaves are thicker and smaller. Shade leaves are larger and thinner (Larger leaves have a greater surface are for receiving sunlight).

Leaf Adaptations For Environmental Extremes Xerophytes are plants that grow successfully in dry climates. They have leaves designed to conserve water, store water and insulate against heat. To inhibit water loss, some have sunken stomata, thick cuticles and sometimes a multilayer of epidermis. Many fibers are in the leaves of xerophytes that help to support the leaf and keep it form losing its shape when it dries. is sunken in an Oleander Plant. in a fig plant. are tightly packed to inhibit evaporation in a jade plant. Jade (Crassula sp.)

Hydrophytes grow in moist environments. They often grow wholly or partly in submerged water. Their leaves are thin and they possess a thin cuticle and the leaves are often deeply loped. Hydrophytes lack characteristics to conserve water. Mesophytes grow in moderate environments (moderate moisture and temperature). They often have typical dicot leaves.

Stipule – a leaf-like structure from either side of the leaf base. Stipules are leafy appendages usually in pairs at the base of the petiole. These are NOT always present. Leaf Modifications For Special Functions Spine – sharp pointed modified leaves that help plants deter predators. Example: Cacti with spines. Black Locust

Tendril – Can either be a leaf or stem. Tendrils are slender coiled organs that aid in the support of stems. In some plants, the entire leaf becomes a tendril. Example shown: Trumpet Creeper climbing another plant using tendrils. Bulbs are modified stems but with fleshy leaves present.

Leaf Abscission is the Seasonal Removal of Leaves Abscission – the process of leaves separating from a stem. Abscission Zone – zone of delicate, thin-walled cells extending across the base of the petiole, the breakdown disjoins the leaf (or fruit) from the stem. In the abscission zone, parenchyma cells are smaller, vascular tissue is shorter and fibers are absent. Leaf abscission in induced by hormonal changes caused by cold or short days. Before abscission, leaves move much of their nutrients to the stem.

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