Morning glory evening grace

Hi, I'm a fan of Hugh Hewitt and many of his callers begin the call with the comment "morning glory" to which Hugh responds "evening grace". He has said often that he would explain its meaning when asked but I'm never around to hear it. Any one care to advise on the origins and/or meaning of "morning glory evening grace"? Thanks.

"Morning" is short for "good morning" --- adding "glory" gives the name of a flower. Responding in kind, "evening" (short for "good evening") is stretched to "evening grace". Got it? Good night nurse.