Page:Collected Physical Papers.djvu/351

 XXIV

THE PHOTOSYNTHETIC RECORDER

The incessant activities of life require expenditure of energy that has been previously stored by the organism. Taking, for example, the rise of sap, the ceaseless pumping activity of certain propulsive tissue raises enormous quantities of water to a considerable height. The energy of doing this work resides in the breakdown of complex chemical substances in internal combustion or respiration. The loss of energy must be restored by absorption and storage of energy from outside.

This is secured in green plants by photosynthesis, carbon being fixed for nutrition of the plant with the help of sunlight.

The carbon-assimilation of plants is of great theoretical interest as an example of the simplest type of assimilation. The plant absorbs the carbonic acid, CO2, and the rate of its intake therefore measures the rate of assimilation. The measurement of assimilation from the intake of CO2 necessitates a complicated chemical analysis, which is therefore a very prolonged and laborious process. It is neither a very sensitive nor a highly accurate method. The following automatic method was therefore devised for recording normal rate of photosynthesis and for quickly indicating any change induced in that rate.

Water-plants obtain their carbon from the carbonic acid dissolved in the water. When sunlight falls on