Yes indeed, it's a lemon tree. Do you remember the old Seekers'/ Peter Paul and Mary song about the lemon tree? I always thought categorising the fruit as impossible to eat was going a bit far, but never mind; it was a metaphor. In this poem, the lemon's many beauties are examined and a cure for envy discovered. To stretch the point to writing, we may endow our major characters with lots of advantages, but we also give them a problem point or so and the readers can think, at least I don't have THAT!