452 Ewart . — The Effects of Tropical Insolation . 
itself playing a perfectly passive part. If a well-defined strip 
of shade is thrown on the median portion of the secondary 
leaf, covering all the pulvini but leaving the leaflets exposed 
to full sunlight, in from two to four minutes the leaflets all 
become horizontally expanded. If now the leaflets are shaded 
but the pulvini exposed to sunlight, the leaflets assume the 
full sunlight position in from one to two minutes. If the 
leaflets, or the pulvini only, over a portion of a secondary leaf 
are shaded or exposed to sunlight, this part only responds. 
If the leaf is in the sunlight-position it is the basal part which 
should be shaded ; but if the leaflets are fully expanded it is 
the terminal part of the secondary leaf which should be ex- 
posed to sunlight, as otherwise the overlapping non-moving 
leaflets interfere with the movement of the irritated portion 
and prevent the formation of the sharp line of demarcation 
between the shaded expanded and exposed folded portions 
which would otherwise be produced. 
When the pulvini alone are shaded, it takes from a half to 
one minute longer for the leaflets to become fully expanded 
than it does when the entire leaf is shaded. If the pulvini of 
the basal pair of leaflets be shaded, the leaflets expand 
horizontally and remain so even though the whole of the rest 
of this pair of leaflets and of the entire leaf is exposed to 
full sunlight. Similarly if one pulvinus only is shaded, that 
leaflet only to which the shaded pulvinus belongs becomes or 
remains expanded. The same is the case with a single leaflet 
or pair of leaflets in all parts of the leaf, but they become or 
remain expanded only as far as the overlapping leaflets will 
allow. 
If the sunlight be intense and the shadow thrown on the 
pulvini narrow, the amount of reflected light that reaches 
the pulvini may be sufficient to cause them to retain a slight 
angle (5 0 to io°) with the horizontal. The interposition of 
a strip of red or yellow glass between the pulvini and the sun, 
causes them to expand nearly fully horizontally, whereas if 
a strip of blue glass is interposed, they remain nearly com- 
pletely folded together. If the leaflets are completely shaded 
