Page:Collected Physical Papers.djvu/355

Rh more and more active until at 1 p.m., the activity increased four times that in the morning. The real reason for increased assimilation at 1 p.m. is the favourable condition of light and temperature. The activity declined in the afternoon and became arrested with the onset of darkness (fig. 105).

One of the most unexpected results, was the, discovery of the extraordinary increase in the power of assimilation by inconceivably small traces of certain chemical substances. It was found that when these were diluted, one part in a billion, they produced an increase in the power of assimilation of more than a hundred per cent. The immediate and concrete demonstration of minute traces of chemicals on assimilation is of special interest, since it enables us to understand the effects of infinitesimal quantities of vitamin on general assimilation and of hormones on physiological reaction.

Formaldehyde, which in large doses acts as a poison, is found in a solution of one part in a billion, to produce an increase of activity of 80 per cent. The stimulating effect of traces of formaldehyde has a special significance in regard to the 'first product' of photosynthesis. There is reason to believe that this first product is formaldehyde, which by polymerisation, becomes converted into carbohydrate. The poisonous nature of formaldehyde stood in the way of acceptance of this view, but the experiment just described shows that traces of this substance, instead of being poisonous, actually enhance the photosynthetic activity.