Ruben and Kamen $(1941)$ and Ruben et al $(1941)$ suspended Chlorella in water having nonradioactive heavy isotope of oxygen, ${ }^{18} O$, instead of natural oxygen ${ }^{16} O$. The suspension was illuminated. Oxygen evolved was tested by means of mass spectrometer. It was found to be heavy isotope, ${ }^{18} O$.This is possible only if oxygen evolved during photos ynth es is comes from splitting of water. The oxygen in glucose comes from carbon dioxide.
$6 CO _{2}+12 H _{2}{ }^{18} O \xrightarrow[\text{chlorophyll,enzymes}]{\text { light }} C _{6} H _{12} O _{6}+6 H _{2} O +6{ }^{18} O _{2} \uparrow$