Modified shoots
A shoot may also change shape to suit its job. Underground shoots: a bulb (onion, tulip, garlic) has a short stem at the base, with modified fleshy scale leaves that store food and dry scales outside; adventitious roots grow from the base. A tuber (potato, Jerusalem artichoke) is the thickened tip of an underground stem, stores much starch and has buds in its “eyes”. A rhizome (Johnson grass, reed, Bermuda grass, mint, licorice) is an underground stem with adventitious roots, scale leaves and buds; it stores food and spreads the plant. These underground shoots carry the plant through unfavourable seasons. Modified above-ground shoots: the strawberry runner, cucumber tendrils, hawthorn thorns. Leaves also change: in cactus and barberry they became spines, the end leaflets of pea became tendrils; the leaves of onion, cabbage and aloe are fleshy and store food.
Practical: with an adult cut an onion lengthwise and find the short stem, fleshy leaves, dry scales and adventitious roots; draw them. Put a drop of weak iodine on a potato slice and watch the starch reaction. Experiment: put an onion bulb on a glass of water and record root and green shoot growth over 7–10 days. Safety: iodine stains skin and must not be swallowed; do not eat iodine-treated potato; knives only for adults; do not rub your eyes with onion juice.