15 MAY 1971, Page 27

Give a dog . . .

Sir: Though John Blackwood and his authority (Letters, 1 May) could be right in their rather un- vivid explanation of the 'dog' in dog's mercury, I remember a de- monstrator in the Oxford botany labs who'd point to the paired fruits of the plant, saying we'd see how the early botanists noticed the resemblance to a dog's bottom.