Abstract
Observations on pure cultures of diatoms and the results of centrifuge experiments showed that under optimal conditions the vegetative cells have the same specific gravity as sea water. The specific gravity of resting spores is significantly higher than that of sea water. Hence the specific gravity of the cell sap which fills the bulk of the vegetative cell and is expelled during formation of the resting spore is lower than that of sea water. The reduced weight (i. e. weight underwater) of resting spores of Ditylum was measured by means of the Cartesian diver balance. The mean value obtained was 0·00312 γ per spore. From calculations of the volume difference between the vegetative cell and the resting spore the specific gravity of the cell sap was estimated at 1·0202, i. e. 0·0025 less than that of the sea water used. It is suggested that the mechanism underlying buoyancy in plankton diatoms is similar to that involved in the flotation of Halicystis and consists in the maintenance of very low concentrations of divalent ions in the cell sap, the result of a steady expenditure of energy.
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