1. A continuously recording glass electrode apparatus has been described for measuring carbon dioxide concentration changes in solution. The limits of applicability of the apparatus have been analyzed.

2. The glass electrode apparatus has been used for the measurement of transient rates of photosynthesis by algal suspensions.

3. The decline of the photosynthetic rate in high light at carbon dioxide partial pressures less than 0.5 per cent atmosphere, observed in the glass electrode apparatus, has been confirmed by steady state experiments in which flowing gas streams were analyzed.

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