218 Blackman and Knight. — A Method of Controlling the 
Giddings 1 controlled the conditions of air movements in some 
transpiration experiments by exposing his plants to the wind produced by 
a fan attached to an electric motor, but he observes that the plants farthest 
from the fan were probably subjected to chance air-currents also ; no 
steps were taken to remedy this. 
The present writers in some unpublished experiments on transpiration 
attempted a more complete control of conditions of air movement than has 
previously been obtained. 
Preliminary experiments were carried out, using a fan in the manner 
described by Giddings. The plants under observation were placed in the 
direct line of the air-current from the fan blades, but, as was to be 
expected, the results were unsatisfactory for the reason mentioned by 
Giddings. The experiments were carried out in a greenhouse, but the 
influence of chance air-currents from doors and ventilators was still con- 
siderable unless the fan was made to rotate at a speed so high that it 
tended to produce rapid transpiration and premature wilting. 
An index to the variations of the rate of air movement was obtained 
by periodical measurements of the rate of evaporation from a paper or 
porous-cup atmometer under constant conditions of temperature and 
humidity. 
To obtain the steady movement of air which was desired, a special 
* air-flue * was constructed, by means of which the plant could be protected 
from chance air-currents, and at the same time could be subjected to 
a current of constant velocity, the velocity being variable at will. The 
apparatus (see figure) consists of a wooden box 2-25 metres long and 
60 cm. in height and breadth. In one end, A, is a circular aperture to 
accommodate the revolving blades of a fan. The other end, B, of the 
flue is open, and the four walls are extended by means of bent metal sheets 
to form a bell mouth, so as to reduce to a minimum the formation of 
eddy currents at the edges of the opening and the consequent irregularities 
in the air movement through the flue. Near the centre of the flue 
a section of the roof and sides is replaced by sheets of plate-glass ; it is 
in this section (c, D), which is 60 cm. long, that the plants under observation 
are placed. The glass sheet at the top is divided in two, parallel to the 
long axis of the flue, and the two parts slide in and out in grooves. The 
front sheet also slides up to facilitate the manipulation of the apparatus 
or plant inside. This sheet is provided with a small sliding door, E, 18 cm. 
square, which is convenient for minor operations not requiring the removal 
of the whole sheet. The woodwork is painted white so that the light may 
not be unduly reduced. Air is drawn through the flue from B to A by 
an electrically driven fan working in the aperture at A. Several fans and 
motors have been tried, but during long-period experiments it was found 
1 Giddings, L. A. : Transpiration of Silphiuni lacinialum . Plant World, xvii, 1914, p. 309. 
