74 The Development of Oxygen by Plants [February, 
much gas given off as at and on further increasing the 
intensity to 8 the quantity of gas eliminated remained 
constant. 
. There is therefore a maximum speed of liberation which 
is reached probably below the intensity i, and on further 
increase of intensity neither rises nor falls. This maximum 
a r caFec ^ *he optimum point, but the light-optimum 
differs from the temperature-optimum by remaining constant 
on further increase of light. 
£P ves the results of his experiments on 
different plants between the intensities i-i6th and 16, in 
seven tables, and sums up the results in the following pro- 
position : ■“ The liberation of gas from Elodea, depending 
upon light, begins at a medium grade of illumination, and 
rises with the increasing intensity of light to a maximum 
which corresponds approximately with direCI sunlight, and 
is reached sometimes at a slightly lower, and at other times 
at a slightly higher, intensity, whilst a further increase of 
intensity produces no further increase of the liberation of 
gas.” 
Heir Reinke next emamined the liberation of oxygen on 
a further augmentation of the intensity of light up to the 
point at which the destruction of the chlorophyll and the 
death of the cells occur. For this purpose a large lens was 
employed, and the distances were calculated for the intensi- 
ve 8 . i (= normal sunlight) up to 64. In order to guard 
against ^the lise of temperature due to the concentration of 
the sun s rays, which would complicate the results, the light 
was passed through a trough 9 inches in width, made of 
plate glass, and filled with a saturated solution of alum. 
Dunng the experiments the rise of temperature did not ex- 
ceed 4 to 6° C. 
. These experiments showed that up to very great intensi- 
ties the number of the gas-bubbles given off was constant 
Nor could any difference in the rate of the liberation of 
bubbles be detected when ordinary sunlight and such light 
concentrated sixty-four times were allowed to fall alternately 
upon the plant. In the most concentrated focal light a 
s P ia y of Elodea continued to give off thirty-eight to forty 
gas-bubbles for a quarter of a minute, as before. Not until 
after an exposure of two minutes did the current begin to 
s A lacke „ n - Fewer bubbles escaped, and at irregular intervals. 
After five minutes the development of gas came to an end 
and on micioscopic examination it was found that a great 
part of the surface of the leaflets was entirely bleached. 
Fiom these experiments it is plain that, even in the most 
