Whodunit: The Chinese Lie detector

When the emperor rose on that April morning, he immediately noticed the silence. "There's no cricket. Where's my cricket?" he demanded. The servants of the bed chamber checked all the usual places, but the cricket was gone—and so was its jeweled cricket box.
The entire royal court was thrown into turmoil until the chirping pet was finally found, housed in a lowly bamboo box and hidden in the corner of a public garden. The emperor was both relieved and outraged. "How dare someone steal from me!" He ordered the captain of the palace guard to find the still-missing box and the culprit.
Finding the box was easy. Lu Ping, a near-sighted gem dealer, bought it from a palace servant and only realized later what he had purchased. With the most abject apologies, he returned it to the emperor. 'I don't know if I can recognize the man who sold it to me," he said with a squint. "I'll do my  best."
From the gem dealer's description, the captain narrowed down the suspects to three. But the dealer couldn't make a positive identification, and none of the three would confess. "Hang them all," the emperor commanded.
That was when the court wizard intervened. "I can discover the guilty servant," he boasted. "Bring the three and come to the public garden."
The wizard led the way to the spot where the cricket had been found. He ordered each suspect to cut a stalk of bamboo. Then the wizard planted the stalks in the hard earth, making sure that each one stuck up the same height from the ground. "By dawn tomorrow," he announced in solemn tones, "the stalk of the guilty man will grow by the length of a finger joint."
By dawn the next morning, the wizard had kept his word. He had discovered the identity of the guilty servant.
How did the wizard do it?

At dawn the next day, the emperor's court returned to the public garden and were amazed to find that not one but two of the bamboo stalks were higher. "Were there two thieves?" the captain wondered out loud."No, just one" the wizard replied. "Arrest the man with the short stalk. He was the only one with a reason to fear my magic. Thinking that his own stalk would grow, he sneaked back here in the middle of the night and cut it a finger joint shorter."