Plants Part 2: Stems

1. Plants Part 2: Stems Text ref 12.3

2. Functions • Connect vascular tissue of leaves to roots • Transport • Raise and support leaves and reproductive organs • Maximize sunlight • Position for pollination • Some are specialized for photosynthesis, storage, protection

3. Structure • Stems can be herbaceous or woody • Herbaceous • Vascular bundles in ground tissue • Contains xylem and phloem • Xylem = closer to centre • Phloem = closer to outside

4. Woody • More complex structure • Vascular cambium – allows for thicker stem growth • Meristematic cells that divide to form new xylem and phloem each year • Xyleminside, phloemoutside

5. Sapwood = younger xylem • Transport • Heartwood = older xylem • Support • Bark = everything out- side of vasc. cambium • phloem, cork cambium, cork • Cork cambium – produces cork • Protection

6. Growth rings • “Spring” wood • Vascular cambium produces xylem rapidly • Thin walls, light colour • “Summer” wood • Vascular cambium produces xylem slowly • Thicker walls, darker colour

7. Vascular Tissue Cell Types • Xylem Cells • Thick walls • Dead at maturity • Cell walls have lignin (strength) • Tracheids = long, cylindrical, tapered ends • Pits (holes) allow transfer of water/solutes • Vessel elements = shorter, wider, less tapered • Sides have pits, ends have perforated plates

9. Phloem Cells • Living at maturity • Contain cytoplasm • Three types of cells: • Sieve cells = pores in walls, have nucleus • Sieve tube elements = lack many organelles • Have sieve plates between cells (transport) • Companion cells = direct metabolic process of sieve tube elements

10. Stem Specializations • Underground stems: • Can form new plant • (tubers, corms, rhizomes) • “On ground” stems • Grow along soil • (spider plants, vines)

11. Homework: • Page 557 # 15