Leaf Surface of Cucumis salivas . 
107 
H. Experiments tinder Artificial Light. 
In order to test this hypothesis more closely experiments were under- 
taken under controlled conditions. These were carried out in the Green- 
house Laboratory of the Imperial College of Science and Technology. 
The temperature of the greenhouse is automatically controlled so that the 
variation in temperature need not exceed 2° F. Within this larger green- 
house is a smaller greenhouse in which the experiments were performed. 
The outside of this inner house was covered with dark cloth, so that the 
inner greenhouse was converted into a dark-room. The walls were hung 
with cloth screens over which water continually trickled, serving to keep 
the atmosphere very humid. A screen of black cloth divided the interior 
of the dark-room into two halves. In one was suspended a single half-watt 
1,500 candle-power lamp, while two similar lamps were suspended in the 
other half. The air within the chamber was kept in continuous circulation 
by means of an electric fan. 
The same variety of cucumber was used as for the previous experi- 
ments. The seeds were graded ; only those falling in the class 37-38 
milligrams were utilized, and were sown singly in small earthenware pots, 
in a mixture of equal parts soil and well-rotted stable manure. 16 pots 
were allotted to each experiment, and these were placed in circles sym- 
metrically about the single lamp and the pair of lamps respectively, the 
distance of the plants from the lamp filaments being 1 metre. 
The seeds were sown on December 10, and germination was uniform, all 
the seedlings appearing on December 14. Leaf areas were determined 
according to the method described, but in these experiments the angular 
measurements as well as the linear were taken each day. Records of 
temperature and humidity were kept by self-recording thermographs and 
hygrographs, and the hygrometer readings were periodically checked by 
means of an Assmann psychrometer, good agreement being found. Except 
for inevitable slight fluctuations due to persons entering and leaving the 
dark-room, both temperature and humidity remained almost constant. The 
average temperature for the period was 95 0 F. (35 0 C), and humidity 
75 per cent, saturation. 
Under such uniform conditions it is perhaps not surprising to find that 
the behaviour of all the plants was identical, all the first leaves appearing 
on December 17 and all the second leaves on December 20. This 
occurred quite synchronously in the case of all the plants, whether under 
the one or two lamps. 
The plants were repotted into larger pots on December 27, and at this 
stage many plants were lost, as they did not recover after repotting. Only 
six .plants out of each set survived, and, as they showed large variations in 
subsequent growth, the experiment was ended on January 4. 
