Leaves - Overview Shoot system Leaf Blade Petiole The leaf is the main photosynthetic organ of most vascular plants Leaves generally have a flattened blade and a stalk called the petiole, which joins the leaf to a node of the stem

Leaves - Comparisons Monocots and Dicots differ in the arrangement of veins, the vascular tissue of leaves Most Dicots have branch-like veins and palmate leaf shape Monocots have parallel leaf veins and longer, slender blades

Leaf Parts 1. Stoma (stomata) – small pores that act as a doorway for gases involved in photosynthesis (found mainly on the underside of leaves). 2. Guard Cells – Cells that regulate the opening of the stomata. 3. Lower and upper epidermis – outer layer of a plant that serves for protection, like skin.

Leaf parts continued… 4. Palisade layer – layer where majority of chloroplasts are found (site of photosynthesis). 5. Spongy layer – cells surrounded by air spaces; allow water, CO2 and O2 to diffuse during photosynthesis. 6. Waxy layer/cuticle – protects leaf from water loss and from feeding insects.

Leaves – Structure and Development Most dicots have 2 types of mesophyll Palisade mesophyll high photosynthesis Spongy mesophyll air spaces for gas & water exchange Monocot leaves have 1 type of mesophyll

Cross Section of a Leaf

Leaves – Structure and Development Leaves are several layers thick – each with different cell types

Epidermis Leaves Leaf epidermis contains stomata - allow CO2 exchange Stomata flanked by two guard cells, control open vs. closed The ground tissue in a leaf, called mesophyll, fills the middle Key to labels Dermal Ground Vascular Cuticle Sclerenchyma fibers Stoma Bundle- sheath cell Xylem Phloem (a) Cutaway drawing of leaf tissues Guard cells Vein Lower epidermis Spongy mesophyll Palisade Upper Stomatal pore Surface view of a spiderwort (Tradescantia) leaf (LM) Epidermal (b) 50 µm 100 µm Air spaces Guard cells Cross section of a lilac (Syringa)) leaf (LM) (c) Figure 35.17 Organization of primary tissues in young stems

Vascular Tissue Xylem – cells that carry water and dissolved minerals from the roots to the leaves. Phloem – cells that carry food (sugar) that is made in the leaves to all parts of the plant. Use: Collectively the vascular tissue, xylem and phloem create a vein or vascular bundle to transport material in plants.

Stoma Open vs. Closed Open – with light and when guard cells are filled with water. Closed – without light and when guard cells are lacking water (dehydrated). Why is it important to have stomata open? Closed vs. Open

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