This is because they can photosynthesize once they develop leaves.
In order for a plant to keep growing and developing, a food source is necessary. They provide energy and raw materials for development.
Inside leaves, there are a lot of chloroplasts. These chloroplasts absorbs light energy to help to plant undergo photosynthesis. This is the reaction where plant makes their own food once their strach supply from seed are used up. They convert carbon dioxide (from the air) and water (from soil) into glucose (sugar) and oxygen (this is a by-product, they're released in air afterwards).
Therefore, as long as the leaves of a plant are exposed to light and has enough water, they can make their own food by a reaction called photosynthesis.
