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90 
ficient to estimate the amounts of chlorophyll present in the 
leaf before and after insolation, the shaded half of each leat 
providing a control, for in all the cases examined whenever 
intense and prolonged insolation caused a permanent stoppage 
of the power of C 0,-assimilation, this was always accompanied 
by a more or less marked destruction of the chlorophyll, the 
chloroplastids turning either yellow or brown, or being entirely 
bleached. (1. c. p. 442). 
Moreover since in sensitive plants prolonged exposure to in- 
tense insolation may cause a stoppage of the function of 
C 0,-assimilation, it follows that an immediate stoppage of this 
function must take place when the plant is exposed to sufli- 
ciently intense illumination, in correspondence with the general 
law that any agency, which when applied in dilute form 
hinders the execution of a particular function or causes its 
gradual cessation, will cause an immediate stoppage when 
applied in sufficiently concentrated form. In other words there 
must be a maximum as well as an optimum light intensity 
for C Q,-assimilation, which may therefore be represented as 
describing a curve with ascending and descending branches 
under an increasing intensity of light. It is possible that in 
very resistant plants the maximal intensity of illumination at 
which C 0,-assimilation ceases to be possible may be so great 
that its stoppage may be followed almost immediately by the 
death of the chloroplastids, but in other cases however, and 
especially when the chloroplastids are much more sensitive 
than the general cytoplasm is, it may be possible to find an 
intensity of illumination at which assimilation immediately ceases 
without any permanent injurious effect being produced upo? 
the plant. Some recent researches by Gitray ') seem to indicate 
that certain plants when exposed to very strong sunlight 
assimilate less CO, or only about the same amount as when 
exposed to weaker light. Even the light which reaches _ 
outermost chloroplastids has already been diminished in intensity 
1) Annales du Jardin Bot. d. Buitenzorg. Vol. XV. 1898, p. 65 (Helianthus) 
