Glucose and starch are the products of photosynthesis. The glucose is used for cellular activities, providing the energy. The remaining unused glucose is stored generally in the roots, mostly as starch. There are also other storage materials. In the sugar cane, it is stored as sucrose, another form of sugar, but it is very sweet. If you ever wondered, that is how sugar canes get their distinctive taste. At night, when the starch collection stops, enzymes break it down into simpler sugars. Oxygen is used to fully gain all energy from the glucose. It comes from the water plants take in, not from the carbon dioxide intaken, as previously mistakenly believed. This was proved by using ‘heavy oxygen’ (18 O), and normal oxygen was emitted.