Hurd and thornley (1974) showed that as Co2 increases from ambient to 0.2%, the relative growth rate (rate of increase in dry weight per unit dry weight) rises more slowly than the net assimilation rate (rate of dry matter production per unit total leaf area). The relative growth rate in enriched plants is soon similar to that of unenriched plant. Co2 concentrations exceeding 0.1% are not recommended as 0.5% Co2 can depress growth relative to 0.1% (Hicklenton and jolliffe,1980).