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R. C. Knight. 
siderably greater than in the dark, but the limits of the method 
would not permit the conclusion that there was no difference at all, 
because although small and fairly consistent differences were 
obtained it seemed likely that they were within the margin of the 
experimental error. This method was accordingly abandoned, and 
more recently the air flue already described (5) has been used for 
experiments along the same lines. Cylinders cut from potatoes 
were suspended in the flue, and the rate of loss of water from 
them was determined by weighing, both under the ordinarily vary¬ 
ing conditions of temperature and humidity in the laboratory, and 
also in the dark room where these factors were constant. In the 
former case various atmometers were used as controls. It has not 
been possible under these conditions to demonstrate any difference 
between the rate of water-loss from the tissue in the dark and the 
rate under the influence of a 1500 candle-power electric lamp 
placed two feet away. Time has not yet permitted a full test of 
the similarity or otherwise between the influence of this lamp and 
that of daylight on stomata and transpiration. A limited number of 
experiments on about six plants (mesophytes and xerophytes) 
showed that the effect of the artificial light on the stomata was 
very similar to that of diffuse daylight. Records of the stomatal 
behaviour of plants in the greenhouse were started, and after a 
time the plant and apparatus were removed to the dark room 
where they remained until the size of the apertures of the stomata 
had become constant. Then the electric lamp was switched on 
and it was observed that the stomata then opened to about the 
same extent as they did in the greenhouse on a dull day. Care 
was taken throughout to prevent the temperature of the air near 
the plant from rising unduly as the result of the heat generated by 
the lamp. To effect this a screen of running water from the tap 
was interposed between the lamp and the plant, and this was found 
to be a sufficient protection if the lamp was about two feet away. 
No tests of the effect of the artificial light on the rate of tran¬ 
spiration have yet been made. 
Experiments have also been carried out, in a manner similar to 
that just described for artifical light, on the effect of diffuse daylight 
on the rate of evaporation from potato tissues, and the indications 
are that there is no difference between the rates of water-loss in 
light and darkness. 
It is proposed next to repeat, under more closely controlled 
conditions, the experiments described by Darwin. The original 
experiments were carried out in a constantly varying atmosphere 
