The Watermelon seed has a fatty coating that provides the germ inside to begin to grow. Once the germ as sprouted and created leaves above the surface, It will begin to undergo photosynthesis, where it will gain the majority of its size and weight from carbon dioxide. Once the plant has grown to a reasonable size it will bloom, the female organ for plants, and if fertilized by another watermelon plant that has created the other part of the DNA for the watermelon, transported by insects, wind, etc. then it will begin to add layers of sugars, created from photosynthesis, onto the plants ovary. That process will continue until either the plant can longer provide more sugar, the watermelon looses its connection to the plant, or the pressure inside the watermelon is greater than that of the evaporation of the water from the plants leaves.