1903.] The Cosmical Function of the Green Plant. 451 



to reflection and absorption by the glass of the tubes at different angles 

 must be taken into account, and for that reason I prefer using in this 

 case these tubes specially prepared for me at the well-known glass 

 manufactory/of Leibold in Koln (Plate 22, B). They are made of plates 

 of mirror-glass luted together. A sample of the same plate is tested 

 photometrically, being placed before a photometer on a divided circle. 



The third method was an application of my micro-eudiometer to the 

 measurement and analysis of the bubbles of gas given off by an aquatic 

 plant, a Potamogeton or Elodea. The quantity of gas produced in 

 1 minute is sufficient for one analysis, so that the same plant in the 

 course of 10 or 12 minutes maybe placed in the different points of 

 the cone of light. Of course this method does not aspire to the same 

 degree of precision as the two former, but its defects are but a con- 

 sequence of the aquatic habits of life of the plant. On the other hand, 

 it has the great advantage of being applied to the same object, all 

 individual differences being eliminated, and the interval of time being 

 so short that one may be sure of the intensity of light remaining 

 invariable during the four consecutive operations. 



On the whole, the results obtained by these different methods on 

 different plants at different times were concordant and maybe summed 

 up] in the form of the following curve 100 being direct sunshine at 

 about noon, the rays falling perpendicularly on the surface of the leaf 

 (% 10). 



1. If the whole range of intensities is taken into account direct pro- 

 portionality is quite out of the question. We may add that Eeinke 

 pushed his experiments still further in the direction of higher intensities. 

 Though he used the old method of counting the bubbles, still for such 

 rough approximations his results may be relied upon. On exposing 

 his plant in a convergent cone of light to intensities amounting to 

 two, four, eight times the intensity of direct sunlight, no corresponding 

 rise of the chemical effect could be observed, the curve remaining 

 parallel to the abscissa. 



2. The maximum effect is obtained, roughly speaking, at about one- 

 half of direct sunlight. 



3. Up to a certain degree of intensity the effect may be considered 

 proportional to this intensity. Past a certain point of the curve, for a 

 further rise of intensity there is no corresponding increase of chemical 

 effect. 



We must now try to find the reason of this general form of the 

 curve. But before entering on a discussion on this subject, we must 

 first take into consideration one point more, concerning another kind 

 of work produced by the radiant energy absorbed by the chlorophyll 

 of the leaf. Professor Wiesner has put it beyond any doubt that a 

 considerable part of the evaporation going on in the green leaf is 

 accomplished at the expense of the radiant energy absorbed by 



