Transpiration in plants
A plant gives off much of the water it takes from the soil as vapour through the stomata of its leaves — this is transpiration. It speeds up uptake of water and mineral salts from the roots, lifts them up the stem and keeps the plant from overheating (like sweating in people). Transpiration is stronger in hot, dry, windy air and weaker on cool, humid, cloudy days. Experiment: if a leafy shoot is put in a flask covered with cotton wool, water droplets appear on the wall. Water use depends on the species: cotton transpires hundreds of times its own weight of water over a summer, one maize plant about 200 litres. Plants of hot dry places save water: saksovul has tiny scale leaves, wormwood has finely divided hairy leaves, in cactus the leaves became spines and water is stored in fleshy stems or leaves (cactus, agave, sarsazan), and some shed their leaves in summer. By their relation to water, ecological groups are: water plants (elodea, water lily), moisture-loving (reed, cattail), plants of moderate moisture (cabbage, clover, maize, wheat, apple) and drought-resistant (saksovul, wormwood, cactus, agave).
Experiment: put a leafy shoot in a jar, add a drop of oil on the water (so that the water itself does not evaporate), mark the level and measure after one day how much has gone (compare indoors and by the window). Discuss: do all plants transpire equally? Safety: hold the glass jar with both hands; do not taste the plant.