Leaves are important because they are the primary source of photosynthesis, which is how plants feed themselves. Photosynthesis is the process of turning light energy into sugars, which plants need to survive, so leaves are very important to a plant's overall health and survival.
If plants can be seen as metaphorical cities, leaves are the farm or food factories of the plant. Green leaves contain chlorophyll, which is not only how the leaves get their green color but also a major ingredient in the photosynthesis process. In addition to energy from light, plants need water and carbon dioxide to make sugars.
Leaves begin in buds, form along the sides of the stem and become visible when they start to unfold. Most soft-stem plants form new leaves until the plants produce flowers or a cold weather season begins. Many woody plants shed old leaves and produce new ones throughout the year. Deciduous plants in temperate regions shed their leaves each fall and produce new ones each spring.