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).