The King's Choice

Riddle: A king has no sons, no daughters, and no queen. For this reason he must decide who will take the throne after he dies. To do this he decides that he will give all of the children of the kingdom a single seed. Whichever child has the largest, most beautiful plant will earn the throne; this being a metaphor for the kingdom. At the end of the contest all of the children came to the palace with their enormous and beautiful plants in hand. After he looks at all of the children's pots, he finally decides that the little girl with an empty pot will be the next Queen. Why did he choose this little girl over all of the other children with their beautiful plants?