Plants produce glucose through a process called photosynthesis.
This process can be summarised with the chemical equation shown below.

Carbon dioxide + water + sunlight => glucose + oxygen

Plants store glucose in the form of starch. Starch is a polymer made by linking glucose molecules end to end. Plants also link glucose end to end to produce another polymer known as cellulose. Cellulose provides the structural strength needed by growing plants to reach huge heights and be able to withstand the forces of nature.
When the plant needs glucose it breaks down the starch into its simple sugar molecules. Un-ripened fruit, such as bananas, contain mainly starch. As the fruit ripens more and more starch is converted into sugar which is why the fruit tastes sweet.
Potatoes are an excellent source of starch. When you chew French fries in your mouth the taste soon becomes sweet. The reason for this is that special enzymes in our saliva convert starch into smaller molecules that taste sweet.

We can identify starch present in foods by adding a drop or two of iodine. A dark blue , almost black, stain appears when starch is present. Click to see a 120kb video.