Photosynthesis happens when the sunlight hits the leaves. This makes the leaves a very important part of making food for the plant. Food and energy Plants make their own food because they have chlorophyll. Photosynthesis is the chemical change that produces food for the plant. In photosynthesis, carbon dioxide, gas and water are mixed to make sugar and oxygen.
Sunlight is the energy it needs for the chemical reaction.
Because of the earth's gravity, the roots grow down and the stem grows up. The gravity is a stimulus. Geotropism is the plant's way of responding. The roots going downward are positive because they are going toward the stimuli. The stem is negative because it is moving away from the stimulus.