Such a fruit is termed a = caryopsis = or = grain =.    From Wordnik.com.  [A Handbook of Some South Indian Grasses] Reference

For one thing, the whole grain (caryopsis) contains a high proportion of germ.    From Wordnik.com.  [6. Pearl Millet: Commercial Types] Reference

As an illustration of the caryopsis, the grain of Andropogon Sorghum may be studied.    From Wordnik.com.  [A Handbook of Some South Indian Grasses] Reference

The caryopsis consists of an embryo on one side at the base and the endosperm occupies the remaining portion.    From Wordnik.com.  [A Handbook of Some South Indian Grasses] Reference