Stephen King is on a roll. His online horror story “The Plant” was so well received — and its readers so honest that they ponied up his $1 charge for the download — that the author plans on moving ahead with a third installment, an assistant to King told Reuters. King had only committed to writing the first two parts in an experiment to test the Internet waters — and the honesty of readers, who were asked to send him the dollar voluntarily. Obviously, they did. “The numbers are very promising,” said Marsha DeFilippo. “He is going to put up Part III.” As of midnight Sunday, “The Plant — Part I” was downloaded 152,132 times, said DeFilippo. The plot involves some greenery that devours a publishing house.