394 Ewart. — The Action of Cold and of 
If a leaf of Elodea is exposed to light in water containing 
carbonic acid, minute bubbles of gas ooze out from the 
ends of the interspaces in the leaf 1 , but when exposed to con- 
centrated light, this ceases almost at once, and instead water 
may be sucked in, either owing to the cessation of C 0 2 - 
assimilation, or to the increased oxidatory activity consuming 
all the oxygen that would otherwise have been set free. 
Reinke 2 found that Elodea gave off gas-bubbles most 
actively in direct sunlight, the activity being unchanged 
even when the light was 60 times concentrated, and the 
evolution of gas-bubbles diminished only when the plants 
were injuriously affected by a much greater intensity of 
light than this. The intensity of the light falling upon the 
outside of a glass vessel is, however, much greater than that 
which penetrates the living cells, and the optimal and 
maximal light-intensities for C 0 2 -assimilation are not those 
which fall upon the outside of the plant, but those which 
reach and react upon the chloroplastids . There is good reason 
to suppose that in all cases the optimal light-intensity for 
continued C 0 2 -assimilation in the chloroplastid is less than 
that of direct sunlight. When plants are exposed to intense 
sunlight in water, the rapid rise in temperature causes the 
intercellular air to expand, and may appreciably affect the rate 
at which gas-bubbles are evolved ; and it is possible, by rapidly 
warming the water in which recently killed plants lie, to 
produce a slight evolution of bubbles from the cut end of 
the stem, just as if feeble C 0 2 -assiniilation had taken place 3 . 
Hence it is doubtful whether the evolution of gas-bubbles 
noticed by Reinke from plants exposed to 60 times con- 
centrated sunlight was actually due to any C 0 2 -assimilation 
taking place at this intensity of illumination. 
It appears therefore that in living chloroplastids exposed 
to light, the decomposition and reconstruction of chlorophyll 
proceed simultaneously, and that in certain cases the total 
1 Cf. Kohl, Ber. d. Bot. Ges., 1897, Bd. xv, p. 120. 
2 Reinke, Bot. Ztg., 1883, p. 713. 
3 See note on Chloroform anaesthetization, infra , p. 415. 
