why is maize grain usually called as a fruit and not a seed?
The maize grain is a fruit in botany and that is for certain.
Now let us see how it confirms to the definition of caryopsis. It is a clear botanical term used in the fruit morphology.
1) Caryopsis is a simple , dry , indehiscent fruit that develops from a superior ovary of a monocarpellary gynoecium
Maize shows all these features as it IS a dry fruit and does not break open automatically on its own so INDEHISCENT fruit. It has a simple flower with a single carpel so MONOCARPELLARY gynoecium.
2) In addition to the above mentioned features. In Maize the TESTA ( seed coat or the wall of the seed ) is inseparably fused with the PERICARP ( wall of the fruit ).
It is called HULL.This is the qualifying character of
CARYOPSIS. This hull shows a tiny scar at the top indicating that the grain is in fact a fruit and not seed.
I hope this will clear your doubt.
All plants that are commonly called 'cereals' belong to Grass family or Poaceae family ( old name Gramineae family)
and they show caryopsis fruit. All of them have endospermic seed and many are invaluable for man.