Vascular plants have a transport system; non-vascular plants such as alga and moss do not. (p.76)
Large plants need vascular tissues to transport water, mineral salts and nutrients to all cells because their surface area is insufficient for direct absorption. (p.76)
Carl Nageli (1858) introduced the names xylem and phloem. (p.76)
Xylem vessels are dead cells without cytoplasm, arranged end to end to form continuous tubes. (p.77)
Xylem walls have uneven lignin thickening that strengthens the vessels against tension and pressure changes and keeps the plant from bending. (p.77)
Tracheids have lignin thickening and pits that allow water to move to adjacent cells. (p.77)
Sieve tubes have no nucleus, ribosomes or vacuoles, and have sieve plates with pores through which sucrose flows. (p.78)
Companion cells contain many mitochondria that supply ATP for active transport of sucrose into sieve tubes. (p.78)
Phloem is living because it has cytoplasm. (p.78)
4.2 Transport of Water and Mineral Salts
Water movement in xylem is helped by transpirational pull, capillary action and root pressure. (p.79)
Transpirational pull is produced when water evaporates from the leaves through stomata and pulls water up the xylem. (p.79)
Capillary action results from adhesion and cohesion forces between water molecules and the xylem walls, moving water upwards against gravity. (p.79)
Root pressure moves water from the soil into the root xylem by osmosis. (p.79)
Water moves from soil into root hair cells by osmosis because mineral ions are actively pumped into the vacuoles, lowering water potential. (p.80)
Osmosis continues across cortex, endodermis and pericycle, and root pressure pushes water into the xylem of the root and stem. (p.80)
Water moves through the root by the simplast pathway (through cytoplasm and plasmodesmata) or the apoplast pathway (through cell walls). (p.81)
Casparian strips block the apoplast pathway, so water must pass through the simplast pathway at the endodermis. (p.81)
Guttation is the secretion of water droplets through special structures at the leaf vein ends, caused by high root pressure, usually at night or early morning. (p.84)
Guttation and transpiration both occur through leaves and cause water loss; guttation releases mineral-rich water through a special structure, while transpiration releases pure water as vapour through stomata. (p.85)
Plants without transpiration may overheat, cannot move minerals such as potassium to the leaves, and may wilt or die. (p.85)
4.3 Translocation
Translocation is the transport of organic substances such as sucrose, amino acids and hormones in phloem from the leaves to other parts of the plant. (p.86)
Translocation supplies roots, fruits, shoot tips and developing flowers, and transports excess products to rhizomes, tubers and bulbs. (p.87)
Sucrose is actively transported from leaf cells into the sieve tube through companion cells. (p.87)
Sucrose entry lowers water potential in the sieve tube, so water diffuses in from the xylem by osmosis. (p.87)
Increased hydrostatic pressure pushes phloem sap along the sieve tube to other organs. (p.87)
Water returns to the xylem by osmosis at the sink, and transpiration pulls water up the xylem to the leaves. (p.87)
4.4 Phytoremediation
Phytoremediation uses plants to degrade, extract or eliminate pollutants from soil and water. (p.89)
Eichhornia crassipes (water hyacinth) has long roots that accumulate heavy metals such as copper and lead. (p.89)
Sunflowers are hyperaccumulators that remove heavy metals and radioactive caesium and strontium from soil, as used after the Chernobyl accident. (p.90)
Pistia stratiotes (water lettuce) grows fast and can accumulate heavy metals and absorb nutrients in waste water. (p.90)
Water spinach roots can absorb mercury from soil and cadmium from water. (p.90)
Phloem removal by ringing causes the upper part of the ring to swell and the lower part to shrink, showing that phloem transports organic substances downwards. (p.88)