Healthy plants tend to create much more food than they can immediately use. The excess food is stored as sugars and starches in various parts of the plants. These stores provide a source of energy not only for the plants, but also for the animals and humans that eat them.
Carbohydrates are the simplest types of foods manufactured and stored by plants. Sugar and starch are two types of carbohydrates. Plant food is made in the leaves, where the green compound chlorophyll absorbs energy from the sun in a process called photosynthesis.
Glucose is a simple sugar that is stored in large quantities in the stems of some plants. One example is the thick stems of the corn plant.
Fructose is another simple sugar. Its chemical composition is slightly different from that of glucose and usually is stored in fruit. For this reason, it commonly is called fruit sugar.
Some plants, such as sugar cane and sugar beets, are very efficient at creating and storing complex sugars. These plants take the simple sugars, glucose and fructose, and create a higher form of sugar that is stored in either the stems, such as in the cane, or the roots, as in the sugar beet.
Starch is a common reserve food in green plants. Unlike sugars, which are soluble in water, starches must be digested before being usable. Starch is stored in grains, such as in rice or wheat plants. Starches are an important staple in the human diet.
The onion bulb that we eat is actually made up of leaves that are specially designed to store water and food sugars underground.