Sunlight upon Aquatic Plants . 395 
amount formed in a single day may be several times greater 
than that present at any given moment. Light acts both as 
a stimulus to the formation of chlorophyll, and at the same 
time induces its photo-chemical oxidation. The former is 
a vital action, and is by no means directly proportional to 
the intensity of the illumination, for ordinary diffuse light 
appears to afford the optimal stimulus to chlorophyll for- 
mation. The decomposing action of light is, however, directly 
proportional to its photo-chemical intensity, if an abundant 
supply of oxygen be present. Hence in strong light the 
decomposition of chlorophyll preponderates, and it is easy to 
understand why an etiolated plant turns green more rapidly 
in diffuse daylight than in direct sunlight. The inhibitory 
effect exercised by strong light upon the assimilatory powers 
of the chloroplastids is not necessarily the result of a dimi- 
nution in the amount of chlorophyll which they contain, but 
may be due to some break having occurred in the chain of 
processes which constitute C0 2 -assimilation, or perhaps to the 
necessary vital connexion between the plasma and the newly- 
formed chlorophyll not being properly established. It is, 
however, inadvisable to push these theoretical conclusions too 
far, nor do these experiments afford any support to Prings- 
heim’s ingenious hypothesis as to the protective function 
of chlorophyll. 
The purpose of my own experiments has throughout been 
to determine how long the living chlorophyll-grains in the 
intact cell can withstand exposure to direct sunlight, and 
what length of time elapses before their functional activity 
is perceptibly affected. Hence when compound tissues, com- 
posed of several layers and covered by an epidermis, were 
employed, the light was frequently concentrated by mirrors, 
or with more sensitive leaves these were simply kept exposed 
to perpendicular illumination for prolonged periods ; while 
when water-plants, & c., were employed, care was taken that 
the full intensity of the illumination actually reached the 
living contents of the cells. 
Once the intensity of the illumination has been deter- 
