iS8 4-1 The Development of Oxygen by Plants. 
73 
II. THE DEVELOPMENT OF OXYGEN BY PLANTS 
AS AFFECTED BY THE INTENSITY OF LIGHT. 
* T_is popularly known that light, and of considerable 
intensity, is needed for that most important process of 
^ plant-life the decomposition of carbonic acid and the 
liberation of pure oxygen. That such an operation takes 
place few will care to dispute. That its results approxi- 
mately compensate for the respiration of animals, though 
not pioved, is generally admitted. But the series of expe- 
riments hitherto conducted with a view to ascertain the 
laws of this phenomenon left much to be desired. Accord- 
ingly the eminent botanist J. Reinke has made a further 
investigation of the subject, the results of which we will 
here summarise. 
As a measure of liberation of oxygen he took the number 
ol air-bubbles thrown off by a water-plant in a given time. 
He threw upon the plant light concentrated by means of a 
lens, measuring the intensity in each experiment by the dis- 
tance of the plant operated upon from the focus of the lens. 
As the subjects of the experiments he selected the points of 
young shoots of the well-known water-weed Elodea Cana- 
densis, which were found to throw off a constant current of 
an -bubbles equal in size. These shoots were placed in well- 
water through which a current of air had been passed for a 
short time. The rays of light were caused to fall normally 
to the surfaces of the leaves. 
Observations were made both with increasing and de- 
ceasing intensities of light. The plant was exposed to 
each degree of intensity for half to one minute before the 
result was taken, and the number of bubbles was then 
counted in two intervals of a quarter minute each. The 
temperature did not rise during the experiments more than 
3 C., and ranged during the different sets of observations 
between 20° and 21 0 C. 
None of the plants examined gave off gas-bubbles at in- 
tensities of light equal only to i-24th of normal sunshine. 
Between the intensities of i-i 6 th and i-qth the number of 
bubbles increased proportionally to the light, — i. e., from 
four to twenty. A corresponding, but of course inverse, 
phenomenon was observed in the decreasing series. But at 
the intensity 1 (= normal sunlight) there was only twice as 
VOL. VI. (THIRD SERIES.) G 
