When young children ask parents, where do babies come from? rather than get into the area of 'the birds and the bees' at such an early age, the stock reply is, the stork brought you, or, we found you under a cabbage leaf.
Here, the author is describing his/her mother as treating him/her as still some kind of baby, so that if the author asked his mother, where do babies come from, she would still regard him/her as a child, her baby still, and so sweet, young and innocent as to tell him she found him/her under a cabbage leaf. The author is humourously accentuating that he/she does not have an adult to adult relationship with his/her mother.
Student or Learner