79 
certain Tropical Trees. 
made at different times of the day with the vertical, were 
observed. The measurement was made by affixing a short 
glass thread to the leaf, so that it projected about half an inch 
beyond and in a line with the midrib. This thread, whose 
point was blackened, played over the flat surface of a vertical 
semicircular arc of cardboard, whose radius was the distance 
between the point of insertion of the leaf and tip of the 
pointer, and which was graduated in degrees 1 2 . 
Plant in room, diffuse light. 
10-50 a.m. 
Thursday 
leaf 
4 0 above horizontal. 
2 p.m. 
„ 
12-5° below 
yy 
2 * 3 ° „ 
yy 
„ 
r 5 ° » 
,j 
4 
yy 
* 7 - 5 ° » 
yy 
5 
V) 
y y 
25 ° M 
yy 
6 „ 
>> 
yy 
30 ° „ 
yy 
8-30 „ 
y. > 
32 - 5 ° y y 
yy 
6-15 a.m. 
F riday 
yy 
IO° „ 
y y 
8 
>> 
yy 
7 * 5 ° » 
yy 
10-40 „ 
)) 
4 » 
yy 
II-IO „ 
„ 
yy 
7 - 5 ° 
yy 
2-45 p.m. 
?> 
,, 
24° „ 
yy 
3 
yy 
>> 
2 7 ° „ 
yy 
3*55 ». 
> > 
„ 
30 ° „ 
yy 
6-15 „ 
5? 
35 ° „ 
yy 
8-30 >, 
y y 
Saturday 
„ 
40° » 
yy 
6-30 a.m. 
>y 
35 ° » 
„ 
8*25 >5 
,, 
„ 
45 ° » • 
yy 
4.35 p.m. 
„ 
n 
5 o° „ 
yy 
3 >> 
„ 
fy 
57 * 5 ° >, 
yy 
These numbers show (i) that a leaf makes a larger angle 
with the horizontal, i.e. is more depressed, when exposed to 
direct sun than when in diffuse light ; (2) that this is brought 
about, first, by the direct effect of sunlight on the periodic 
movement, and, secondly, by indirect after-effect, whereby the 
nocturnal recovery tends, in leaves which have been exposed 
to direct sunlight, to be inhibited. This latter indirect effect 
of sunlight explains the fact that young leaves of Brownea 
grandiceps growing in the sun seem to hang permanently 
downward. 
1 See Darwin, Power of Movement, pp. 330, 331. 
2 At this point the plant was put in the open, but protected from rain and wind. 
The day was cloudy with but occasional sunshine. 
