1 Review Describe how the structure of the leaf is adapted to make photosynthesis more efficient Form a Hypothesis The leave of desert plants often have two or more layers of palisade mesophyll rather than one. How might this modification be advantageous to a desert plant 2 Review How do stomata help plants maintain homeostasis Predict Are stomata more likely to be open or closed on a hot day- explain

Ch 23 Plant Structure and Function 23.4 Leaves

Anatomy of a Leaf Blade Petiole Thin, flattened part of leaf to collect sunlight Petiole Thin stalk that attaches stem to blade Have dermal, ground, and vascular tissues.

Dermal Tissue Epidermis is covered by a waxy cuticle that protects the leaf and limits water loss through evaporation Tough irregular cells with thick cell walls.

Vascular Tissue Xylem and phloem tissues are gathered together into bundles called leaf veins.

Ground Tissue Palisade mesophyll Spongy mesophyll Closely packed cells that absorb light that enters the leaf Spongy mesophyll Has many air spaces between its cells.

Stomata Small openings in the epidermis Allow carbon dioxide, water, and oxygen to diffuse into and out of the leaf Connects to spongy mesophyll.

Transpiration Loss of water through leaves due to evaporation Replaced by water drawn into the leaf through xylem Mesophyll cells are kept moist so that gases can enter and leave the cells easily Helps to cool leaves on hot days.

Gas Exchange Leaves take in carbon dioxide and give off oxygen during photosynthesis Cells take in oxygen and give off carbon dioxide during cellular respiration Gas exchange uses stomata.

Stomata always open Stomata always closed Water loss would be so great, few plants would be able to survive Stomata always closed Air exchange couldn’t occur Keep stomata open just enough to allow photosynthesis to occur but not enough that they lose excessive water.

Guard Cells Highly specialized cells surround stomata Control their opening and closing Regulate gases exchange.

Open Stomata Carbon dioxide can enter Water is lost by transpiration.

Open Stomata When water is abundant, it flows into the leaf, raising water pressure in the guard cells, which opens the stomata.

Closed Stomata Carbon dioxide cannot enter Water is no lost by transpiration.

Closed Stomata When water is scarce, water pressure within the guard cells decreases, the inner walls pull together, and the stoma closes.

Homeostasis Stomata open during the day for photosynthesis, and close at night to limit water loss May close in bright sunlight under hot, dry conditions to conserve water.

Transpiration and Wilting Osmotic pressure keeps a plant’s leaves and stems rigid High transpiration rates can lead to wilting.

Adaptations of Leaves Pitcher Plant: Leaf is modified to attract and digest insects and other small Typically live in nutrient-poor soils and rely on animal prey as their source of nitrogen.

Rock Plant Two leaves are rounded to minimize exposure to air Have few stomata.

Spruce Narrow leaves contain waxy epidermis and stomata that are set below the surface of the leaf Reduce water loss.

Cactus leaves are nonphotosynthetic thorns Protect against herbivores Photosynthesis carried out in stem.

Examining Stomata Obtain different kinds of leaves from your teacher Spread a thick coating of clear nail polish on the underside of each leaf Wait about 10 minutes for the polish to dry completely Attach a strip of clear tap to the polish and gently peel off the tape and the dried polish Tape the polish to a clean microscope slide and examine under a 40x lens For each leaf, move the microscope stage so you can count stomata from three distinct fields of view

Examining Stomata Calculate What is the average number of stomata per square cm for each leaf Graph Make a graph that compares these averages Form a Hypothesis Write a hypothesis that could account for differences in stoma density among plants