Sunlight upon Aquatic Plants . 393 
difference is perceptible. This is evidently due to the fact 
that the reconstruction of chlorophyll is more active in 
the younger chloroplastids, and it is easy to see why chloro- 
phyllous organs, when the sunlight is discontinuous, may be 
able to withstand a far greater total exposure than when 
it is continuous. 
It is of considerable interest to know whether there is 
a certain maximal intensity of light beyond which C 0 2 - 
assimilation immediately ceases to be possible, so that 
momentary exposure to such light acts just as momentary 
darkness does, causing assimilation to cease until normal 
conditions of illumination are restored. Intense illumination, 
however, soon puts the assimilatory mechanism out of order, 
and hence a direct stoppage of C 0 2 -assimilation is possible 
without the chloroplastids being markedly affected, only when 
the exposure is extremely brief. The following experiments 
were all performed with cooled and 10 times concentrated 
sunlight. The bacterium-method is not directly applicable, 
for any Bacteria which come within the circle of intense 
light are at once rendered im motile, and moreover a closed - 
cell-preparation must be employed. After 5 minutes’ ex- 
posure in a drop of water the power of assimilation is lost 
by the exposed parts of an Elodea leaf, and may either 
not return, or, if the chloroplastids are still green, recovery 
may take place in one hour to a day. In a closed-cell- 
preparation, rotation may be stopped and may never recom- 
mence if the entire leaf has been exposed, the leaf-cells dying 
of asphyxia in 2-3 hours, whereas if oxygen is admitted, 
rotation recommences and the cells may remain living. The 
chloroplastids have therefore temporarily or permanently 
lost the power of producing oxygen ; for in a normal closed- 
cell-preparation exposed to light of moderate intensity, the 
oxygen evolved by C 0 2 -assimilation suffices to maintain 
active rotation, the latter ceasing after 10-15 minutes’ dark- 
ness, and recommencing within a minute in the light, though 
it does not become fully active until several minutes have 
elapsed. 
