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Every year afterwards new mistletoe "roots" are formed which grow through the soft part of the bark and send down sinkers into the wood. Cases of Mistletoes forty years old have been recorded. The trees which they prefer are the Apple, and after that Black Poplar, though mistletoe may be found on Silver Fir, various Pines, and others. It is more difficult to get it to grow on the Oak than on any other tree. Indeed, only seven cases of mistletoe growing on oak have been recorded in this country.[144] It is quite a valuable crop in some places, and is sent in tons to the London market.