20世纪上半叶,美国密歇根州比拉镇附近的农场里,主人养了一头公牛,他用铁链把牛锁在了一棵榆树上。健壮的公牛经常拖着沉重的铁链围着榆树奔跑,日复一日,铁链在距离地面约一米的树干上勒出了一道沟痕。数年之后,铁链已深深地嵌在了树皮中,和树干牢牢地长在了一起。后来有一年,密歇根州遭到“荷兰榆树病”的袭 In the first half of the 20th century, the owner raised a bull in a farm near Pila town, Michigan, and locked the bull with an iron tree in an elm tree. Sturdy bulls often drag heavy chains around the elm trees, day after day, chains on the trunk about a meter from the ground leaking a trench. Years later, the chains were deeply embedded in the bark and grew firmly with the trunk. One year later, Michigan was hit by “Dutch elm disease.”