Stephen King has pulled the plug on “The Plant,” his self-published online serial novel, because too many eager readers are downloading the work without paying for it. In a statement published on his Web site, the prolific horror novelist told readers that after next month’s installment he’ll put aside “The Plant” for a year or two so he can focus on other projects. (These include finishing two novels and co-writing “Black House,” the sequel to “The Talisman” with Peter Straub). Besides, wrote King, “my agent insists I need to take a breather so that foreign translation and publication of ‘The Plant’ . . . can catch up with American publication.” (Both versions are appearing in installments on the Net.) December’s final installment of “The Plant” — part six, of what was to have been a 10-part series — will be free, King promises, “as a way of thanking those readers (somewhere between 75 and 80%) who came along for the ride and paid their dues.” King had been offering his story to readers on an honor system on a chapter-by-chapter basis for one or two dollars, depending upon the installment.