Page:Collected Physical Papers.djvu/409

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In the median range of the photosynthetic curve the increase of activity is proportional to the intensity of light. The latter is determined by the Self-recording Radiograph (p. 338). As regards the effect of rise of temperature, the increase of photosynthetic activity is uniform in the median range of temperature variation, there being an abrupt decline beyond the optimum. The combined effects of the factors of variation of light and temperature, explain the hourly variation of photosynthetic activity which is at its maximum at about 1 p.m. (p. 334).

Investigations on the effect of infinitesimal traces of certain chemical substances show that they produce a very great increase in the activity of assimilation. The demonstration of this is of special interest since it enables us to understand the effect of infinitesimal quantities of vitamin on general assimilation and of hormones on physiological reaction.

The effect of minute quantity of formaldehyde in enhancing the activity is of special significance in regard to the possible formation of formaldehyde as the first product of photosynthesis. This substance is toxic only in a strong dose; before there could be any great accumulation of this substance in the cells it would have become polymerised into carbohydrate.

The estimates hitherto obtained indicate generally a very low efficiency; the experimental methods employed in this determination have been defective from absence