BOTANY OF THE CEYLON PATANAS. 327 
oils is a very common character of dry-climate plants (43). 
The explanation usually given of the function of such an oil 
is founded upon Tyndall’s observation that radiant heat is 
arrested by minute quantities of the vapour diffused through 
the atmosphere, and is to the effect that the evaporation of the 
oil by the heating effect of the sun's rays causes the air 
surrounding the leaf to be charged with vapour, which acts as 
a screen protecting the leaf to some extent from radiant heat 
from without (44) It has, however, been pointed out by 
Dixon (45) that such a screen absorbs those heat- -rays which 
it does not transmit, and that its temperature is thereby raised, 
and it thus encloses the leaf in a heated chamber; therefore 
the mere physical effect of an ethereal oil-vapour screen would 
be to raise, rather than lower, the temperature of the leaf. 
Dixon has further shown (45) that certain ethereal oil-vapours 
act biologically in deereasing the rate of transpiration when 
they are in contact with transpiring leaves; in the case of the 
essential oil of Artemisia Absinthium, he found that the vapour 
reduced the rate of transpiration by 13 °/, in the leaves of 
Syringa vulgaris, and by 7 °/, in those of Cytisus vulgaris. Tt 
may be that “ Citronella” oil-vapour has a stronger effect upon 
the rate of transpiration of the leaves of Mana grass than is 
represented by these figures ; otherwise it is diffieult to believe 
that so small a reduction effeeted by an agent which itself 
raises the temperature of the leaf, and therefore presumably 
its rate of transpiration, can be of much service to the plant 
as a means of regulating the rate of transpiration. Mana 
grass has a marked gregarious habit, generally cov ering wide 
patehes of ground below 5000 feet, where the slope or other 
quality of the surface has allowed an unusual accumulation 
of soil. In the heat of the day the Citronella perfume can be 
detected at some distance from its source, and it is conceivable 
that if it has an effect in reducing transpiration, of the nature 
indicated, the effect must be felt by the plants in the neighbour- 
hood, as well as by the Mana grass itself. 
A permanent erect profile position of the leaf is very common, 
and this applies not only to many patana-plants, but is also 
characteristic of very many of the trees which compose the 
montane forest above 5000 feet. In very many more eases, 
however, the leaf makes a small angle (7. e. less than 45°) on its 
upper side with the stem ; incident light strikes a leaf in this 
