Calvin cycle (C3 cycle) was discovered by Calvin, Benson and their associates when fed Chlorella and Scenedesmus with radioactive carbon (C14) in CO2. They found that after three seconds radioactivity appeared in phosphoglyceric acid or PGA (the first stable product of Calvin cycle). Spirogyra is a free floating freshwater alga. The body is filamentous, uniseriate and unbranched. It shows, sexual reproduction by conjugation. Chlamydomonas is a freshwater, free floating alga. It is unicellular and imparting red colour to snow hence, called red snow. Volvox is a green, flagellate colonial alga of world wide distribution. A colony consisting of a definite number of cells arranged in a specific manner and forming an integrated whole is called a coenobium.