93 
certain Tropical Trees. 
to a coarse and fine adjustment — to use a comparison sug- 
gested to me by Mr. Francis Darwin. The coarse adjustment 
is effected by the rising up of the petiole, the fine adjustment 
by the periodic movement of the leaflets. These latter also 
have the power of temporary adjustment from day to day 
before and during the uprising of the petiole. 
During the hanging of the leaf, when the permanent 
conditions to which the branch will be exposed are being 
arrived at, the power of the leaf to assume any position within 
a range of nearly i8o° is possible; and this enables every 
leaflet of the tree to arrive at its best possible position with 
respect to insolation. 
In addition, then, to the protective value suggested by 
previous writers, which has been demonstrated in the fore- 
going paper, I would suggest that the habit under examination 
has an additional value of adjustment ; whereby a greater 
‘ choice ’ of positions is at the disposal of branch and leaf. 
Although hunting for teleological explanations may not 
in itself be a productive task, yet, in so far as these 4 ex- 
planations ’ open up new points of view, they are, I venture to 
submit, the just objects of biological research: thus I would 
justify the line of argument employed in this paper. 
In conclusion, I wish to acknowledge the great kindness 
shown to me by the Director of the Royal Botanical Gardens 
of Ceylon, Dr. Trimen, who granted me permission to make 
use of the gardens and of the laboratory. I also beg to 
express my thanks to Mr. Francis Darwin, who has aided 
me in the writing of this paper with much valuable criticism. 
