Leaves Take it or leaf it!

Leaf Shape Usually flat and thin to maximize solar energy capture. Extremely variable, adapted to environmental conditions. Blade – broad flat part, most photosynthesis happens here. Petiole – attaches leaf to stem (may be absent). Simple leaf – single blade. Compound leaf – blade divided into leaflets. Margins (edges) vary greatly. Leaf Shape

Leaf Structure – Dermal Tissue Epidermis covered by waxy cuticle (protects against water loss. Water vapor, CO2 & O2 enter and leave leaf through stomata, usually on underside of leaf to prevent clogging by dust or rain. Epidermal hairs protect leaf from insects, intense light, drying out.

Leaf Structure – Ground Tissue Mesophyll – where most photosynthesis occurs. Palisade mesophyll – upper layer, columnar, fairly tightly packed but with some air spaces. Spongy mesophyll – lower layer, cells irregular in shape, lots of air spaces so gases diffuse into and out of leaf easily.

Leaf Structure – Vascular Tissue Veins have xylem and phloem. Embedded in mesphyll. Veins branch so every cell is close to one. Parallel venation – major veins parallel to each other. Net venation – veins repeatedly branch into varying patterns.

Leaf Functions Usually the main site of photosynthesis. Also use light energy to make amino acids, fats, other organic compounds. Transpiration draws water up stem, cools leaf, speeds up mineral transport from roots.

Leaf Function Various adaptations for maximizing light: Arrangement on stem minimizes shading of lower leaves. Leaves on sunny side (sun leaves) – thicker, smaller surface area, more chloroplasts, stomata. Shade leaves thinner, often larger, arrange chloroplasts carefully to minimize shading each other. Desert plants may need to minimize light intensity and drying with dense hairs & thicker cuticles. Leaf Function

Plants need CO2 & water vapor for photosynthesis, O2 for cellular respiration. Regulate transpiration with guard cells bordering stomata. Stomata open during day, close at night, controlled by turgor pressure (amt. of water) in guard cells. Open - epidermal cells pump K+ ions into guard cells, water moves in by osmosis, they swell and bow apart. Reversed at night. Drought – stomata close, restricts transpiration. Downside: less water moving in from roots and less CO2 in from air = less photosynthesis, plant cells and tissues wilt, leaves may overheat and die. Leaves in drier habitats have fewer stomata. Gas Exchange

Specialized Leaves Cactus spines protect plant, reduce water loss via transpiration. Tendrils for support & climbing. Food traps – carnivorous plants grow in soil poor in mineral nutrients, esp. nitrogen – trap and digest animals.