Metabolic wastes or
excretes are
substances left over from
excretory processes, which cannot be used by the
organism (they are surplus or have lethal effect), and must therefore be
excreted. This includes
nitrogen compounds,
water,
CO2,
phosphates,
sulfates, etc.
Animals treat these compounds as excretes.
Plants have
chemical "machinery" which transforms some of them (primarily the nitrogen compounds) into useful substances, and it has been shown by
Brian J. Ford that
abscissa leaves also carry wastes away from the parent plant. In this way, Ford argues that the shed leaf acts as an
excretory (an organ carrying away excretory products).