Roots, stems, and leaves
Roots Absorb water and dissolved nutrients Anchor plants in the ground Holds soil in place to prevent erosion Protect from harmful soil bacteria and fungi Transport water and nutrients to the rest of the plant Hold plant upright against forces such as wind and rain
Types of roots Taproot Primary root grows long and thick while secondary roots remain small Some store sugars and starches Hickory, oak, carrots, dandelion, radish Fibrous root Branch so that no single root grows larger than the rest Help prevent topsoil from being washed away Grasses, rye,
Root structure and growth A mature root has an outside layer, epidermis, and a central cylinder of vascular tissue Ground tissue fills the area between Surface covered with root hairs Produce a larger surface area for water absorption root grows in length at tip Root cap protects fragile new cells as it forces its way through the soil
Stems Support system for the plant body Must be strong enough to hold up leaves and branches Holds leaves up to the sunlight Transport system that carries nutrients Must be able to lift water from roots to the leaves Defense system that protects plant against predators and disease Produce leaves, branches, and flowers
Stem structure Surrounded by thick cell walls and waxy protective covering Nodes- where leaves are attached Internodes- regions between nodes buds- undeveloped tissue that can produce new stems and leaves Found where leaves attach to nodes
Stem growth Primary growth- new cells are produced at the tips of roots and shoots Secondary growth- must increase in thickness as well as length Takes place in lateral meristems Vascular cambrium, cork cambrium, and cork Vascular cambrium- produces vascular tissue and increases the thickness of stems over time Cork cambrium- produces outer coverings of stems
Formation of wood Actually layers of xylem Heartwood- old nonfunctioning xylem Helps support the tree Usually darkens with age Sapwood- surrounds heartwood active in fluid transport Usually lighter in color Tree rings formed from alternation of dark and light bands formed from early and late wood during the growing season Each ring corresponds to a year of growth
Formation of bark Bark includes all of the tissue outside of the vascular cambrium Phloem, cork, and cork cambrium As new xylem is produced phloem must grow to accommodate the larger size of the tree Phloem is pushed outward and the expansion causes it to split and fragment The cork cambrium surrounds the cortex and produces a thick protective layer of cork
leaves Main photosynthetic systems Optimized for absorbing light and carrying out photosynthesis Broad, flat surfaces help increase the amount of light absorbed
Leaf structure Blades- thin flattened section that collects sunlight Petiole- thin stalk that attaches leaf to stem Cuticle and epidermal cells form waterproof barrier Vascular tissue of leaves connect directly to the vascular tissue of the stems
Leaf functions Photosynthesis Bulk of leaf is mesophyll Site where photosynthesis occurs Palisade mesophyll- absorbs light Stomata- surrounded by guard cells, allow gas exchange Transpiration-the loss of water through leaves Replaced by water drawn into the leaf through xylem Surface of leaf kept moist for gas exchange
Leaf functions cont. Gas exchange Take in carbon dioxide and give off oxygen during photosynthesis Take in oxygen and give off carbon dioxide during cellular respiration Stomata- site for gas exchange Open just enough to allow photosynthesis to take place, but not so much that they lose an excessive amount of water Usually open during the daytime and close at night Guard cells- control opening of stomata Stomata open and close in response to changes in water pressure within guard cells