Ewart . — The Effects of Tropical Insolation. 451 
If a layer of cold water is interposed between the sun and 
the exposed leaves, the leaflets rise up as usual but not quite 
so markedly (5 0 to io° less), the difference being due probably 
to the slight loss by refraction and dispersion as the light 
passes from the air through the optically denser medium and 
out again. If sheets of coloured glass are interposed, in all 
cases the leaflets do not rise up so much as when exposed to 
direct sunlight ; under blue glass the difference being least, 
under yellow more, and under red glass most marked. When 
exposed to intense sunlight under blue glass of not too great 
thickness, the leaflets may fold up nearly to their full extent ; 
whilst when a double layer of thick red glass is interposed 
they may expand fully horizontally. Sachs and Bert have 
shown that in red or yellow rays from light of only moderate 
intensity the leaflets fold together, assuming the nyctitropic 
position, whereas in the blue rays from light of similar intensity 
they remain expanded. Hence when the blue rays are weak 
or absent the leaflets fold together, and when the rays are too 
intense they again fold together. It is obviously an advantage 
that the blue rays, when too intense, should cause the leaflets 
to fold together and hence be protected from excessive photo- 
chemical action ; but that blue light of only moderate intensity 
should cause the leaflets to pass from the nyctitropic into the 
expanded position is only accidentally and indirectly an 
advantage, owing to the fact that under normal exposure the 
intensity of the blue rays is a sufficiently accurate measure 
for the plant of the intensity of the accompanying rays 
absorbed by the chlorophyll and used in assimilation. The 
leaflets respond not to the heat but to the light-rays of 
the sun, and of these rays it is the more refrangible ‘ chemical ’ 
rays which are most effective. 
The following experiments show clearly that not only is 
the pulvinus the motile organ, but it is also the irritable 
perceptive organ and directly responds to the stimulus exerted 
by a change in the intensity of the illumination, the leaflet 
1 Sachs, Bot. Zeitg., 1857. Bert, Mem. de la Soc. d. Sci. de Bordeaux, VIII, 
1870. 
