A Method of Controlling the Rate of Air Movement 
in Transpiration Experiments. 
BY 
V. H. BLACKMAN 
AND 
R. C. KNIGHT, 
From the Department of Plant Physiology and Pathology , Imperial College of Science 
and Technology, London. 
With one Figure in the Text. 
T HE influence of air movements on the rate of transpiration of plants 
has long been recognized but attempts to control this factor during 
experiments on transpiration have been very few. Investigators have 
usually contented themselves with periodical observations of the speed of 
the air in the vicinity of the plant, or else have attempted to eliminate from 
their results the effects of air-currents and other external factors by directing 
their attention, not to absolute transpiration, but to c relative transpiration 
i. e. to the ratio between the rate of transpiration from the plant and the 
rate of evaporation from an atmometer. Such a method is, however, not 
altogether satisfactory since the response of the plant and that of the 
atmometer to air-currents are not proportional, as has been demonstrated 
elsewhere . 1 
On the other hand, attempts to obviate the effects of air-currents by 
investigations of transpiration in ‘ still air ’ are equally unsatisfactory, on 
account of the difficulty of ensuring that the air is really still. Experience 
has shown that in spite of precautions that may be taken the air of 
a laboratory is in constant slight motion, and such chance air-currents will 
influence the ‘shells’ of water vapour over the evaporating surfaces and 
consequently the rate of transpiration will continually vary. 
In critical work on transpiration it would therefore seem advisable 
that work should be carried out neither in ‘ still air nor in the open where 
the wind currents may vary with great rapidity, but under conditions of 
constant air movement which can be regulated at will. 
1 R. C. Knight, in a paper appearing later in Vol. XXXI. 
[Annals of Botany, Vol. XXXI. No. CXXII. April, 1917.] 
