But seed orchards are difficult to control, because tree pollen spreads so far that alien, non-elite trees almost inevitably muddy the gene pool.是什么意思_But seed orchards are difficult to control, because tree pollen spreads so far that alien, non-elite trees almost inevitably muddy the gene pool.在線(xiàn)翻譯_英語(yǔ)_讀音_用法_例句