certain Tropical Trees. 65 
withered, the green ones similarly placed were not; nor were 
either of the two shaded sets of leaflets. 
Indeed the experiments were repeated so frequently as 
to leave no doubt that, under similar conditions of exposure, 
the thin delicate immature leaves of Amherstia nobilis wither 
more rapidly than the more fully developed leaves ; that, 
as would perhaps be surmised, with increasing toughness and 
thickness there occurs an increase of resistance to too great 
a loss of water — together, may be, with increased facility in 
obtaining water ; that under unfavourable conditions the 
younger leaves are more readily damaged than the older. 
Since the young leaves of Amherstia nobilis are readily 
injured by exposure to direct sunlight, at all events under 
the conditions of the above experiments, the opposite con- 
clusion to that at which Stahl arrives might be urged — viz. 
that such risk of injury indicates a need, on the part of the 
tender foliage, of special protection, and that the hanging 
position is an adaptation to this end. 
It does not, however, follow that the habit under examina- 
tion was developed solely as such a means of protection. 
It is possible at least to suppose that this habit has some 
other, or, at all events, additional significance. It seemed 
worth while to compare the rates of transpiration of the 
mature and immature leaves. 
The comparison of the rates of transpiration was generally 
made by aid of the potometer 1 . By this method is 
measured, not the rate of transpiration, but the rapidity with 
which water is taken up by the branch. 
Branches were cut at 9.30 a.m. and brought to the apparatus. 
The following numbers give the time in seconds that bubbles 
took in passing from one mark to the other on the capillary 
tube of the potometer, the bubbles being made to follow one 
another as rapidly as possible. 
1 Darwin and Phillips, Cambridge Philosoph. Soc., Vol. v, 1886. 
F 
