food storage in plants
Some plants grow from seed, flower, and produce more seed all in one season. These are annuals and they do not store much food because it is used up straight away in forming new tissues. Other plants, however, live for two or more years. Except in tropical regions, there is a definite growing season. In winter, deciduous trees drop their leaves and the above-ground parts of many herbaceous plants die down. They must, however, have stored sufficient food to enable them to start growth in the spring. Trees store food in the tissues of the trunk and branches but many herbaceous plants have special underground storage organs. These may be modified roots or stems.
Storage in swollen tap roots is common in biennials (plants that grow and store food one year and flower and die the next). The carrot is an excellent example. By digging up the carrots at the end of the growing season, humans make use of the food that would have gone to make the next year's growth (see upper illustration).
Root tubers are found in dahlias and the lesser celandine, for example. They develop from tiny buds at the base of the plant. They swell as food is passed into them and remain in the ground after the aerial parts have died down. Each tuber and its bud can give rise to a new plant.
Underground stems are the most common storage organs. They vary in structure but differ from roots in the possession of scale leaves and buds. Rhizomes are horizontal underground stems and are found in irises and many grasses. They do not always store food but when they do they are quite thick. The potato is a stem-tuber – a swollen part of a stem that stores food. When detached from the rest of the plant, the potato acts as a reproductive body.
Corms and bulbs are both underground storage structures and are often confused. In corms, the food reserve is stored in the swollen stem, while in bulbs, swollen scale leaves or the swollen bases of the previous year's green leaves contain the food (see lower illustration).
Seeds are, of course, supplied with food reserves that enable the young plant to establish itself until it can begin to manufacture its own food.