Be Able To:
1. Identify the names and functions of leaf tissues.
2. Identify the names and structures of stem tissues.
3. Identify two types of roots and three root functions.
4. Define tropism and give examples.
The LEAF is an organ made of tissue layers, and its main function is PHOTOSYNTHESIS. The top layer is called the CUTICLE, and it is a transparent waxy (lipid) coating that prevents dehydration. The next layer is the UPPER EPIDERMIS, and it is made of rigid transparent cells that keep the leaf spread out so there is lots of surface area for sunlight to reach. The next layer is the PALISADE layer, made of vertical cells with lots of chloroplasts, because this is where most of the photosynthesis occurs for the plant. All of the layers above this are transparent so sunlight can reach the chloroplasts in the palisade cells. The next layer is the SPONGY LAYER, where gases used in photosynthesis are stored (such as O2, CO2, and H2O). The bottom layer is the LOWER EPIDERMIS, and it contains two-celled structures called STOMATA, that open and close to allow gases into or out of the leaf. The opening and closing of the stomata depend on turgor pressure.
The STEMS have two functions, support and transport. They support that plant above the ground. They also contain the VASCULAR TISSUE of the plant, which are a series of tube-like structures that transport materials throughout the plant. Small plants like mosses do not have a vascular system, but most plants do. There are two types of vascular tissue. The XYLEM is vascular tissue that brings water and minerals up from the roots to the rest of the plant. The PHLOEM is vascular tissue that brings sugar (glucose) both up and down the plant as needed.
The ROOT is an organ that has three functions. First, it anchors the plant into the ground so it is secure. Second, roots absorb water and minerals from the ground, which are then transported by xylem to the rest of the plant. Third, some roots store starches for energy within their roots- like carrots. A root shape that is thick, long, and deep is called a TAPROOT (found in dicots). A root shape that is thin with lots of fibers that does not go very deep is called FIBROUS ROOT (found in monocots).
A TROPISM is a plants response to a stimulus and is controlled by plant hormones. A stimulus can be the sun, water, gravity, or touch. A POSITIVE TROPISM causes a plant to grow TOWARDS the stimulus. For example, a plant growing towards sunlight would be POSITIVE PHOTOTROPISM. A NEGATIVE tropism causes a plant to grow away from the stimulus.