1 1 8 Gregory . — The Increase in Area of Leaves and 
that it markedly affects the rate of respiration. Now during the winter 
months in the latitude of London the hours of darkness greatly preponderate 
over the hours of light (in December 8-5 hrs. light, 1*5*5 hrs. darkness), while 
the intensity of daylight is at a minimum ; thus photosynthetic assimilation 
will not only be low, but the accumulation of material will be markedly re- 
duced by respiration. With the high temperatures prevalent in greenhouses 
during the winter months this loss due to respiration may be considerable, 
so that the plant may be in a state of semi-starvation The deleterious 
effects of high temperatures are seen in the experiments under artificial 
light. 
Whether the detrimental factor discussed above is to be attributed to 
incipient starvation or to some other unknown factor concomitant with low 
intensity of illumination it is impossible, from the data so far collected, to 
decide. Moreover, there can be no doubt that light has, apart from its 
photosynthetic effect, a definite morphogenic action on the plant, so that 
the elucidation of the complex phenomenon awaits further investigations. 
Fig. 15 shows the log time plotted against log area for plants grown 
under artificial light up to the 13th day, the time of repotting. Here again 
a straight line is obtained, so that, though the curves of total leaf-surface ; 
increase for plants grown in December (Fig. 13) and under artificial light 
(Fig. 10) appear so diverse, the same law is being followed under artificial 
illumination as under low intensity daylight. 
The equations of the curves of total leaf-surface increase are as 
follows : 
December in daylight : A — 0-45 /f 1 * 76 , 
under two lamps: A = 4*06 /°* 692 , 
„ one lamp : A = 3*41 ^°- 413 . 
The different aspect of the curves is now clear, for when the index of t 
is greater than unity the curve will be convex to the time axis, when the 
index equals unity the curve becomes a straight line, and finally as the index 
falls below unity the curve is concave to the time axis. 
In the case of plants grown under artificial illumination a detrimental I 
factor is again seen to be at work. 
Leaf-surface growth under artificial light. The intensities of the 
incident radiation in the two experiments with artificial illumination were as 
follows : 
With two lamps : 0-216 cals, per cm. 2 per min. 
With one lamp : 0-125 cals, per cm. 2 per min. 
It will be seen in Table V that these intensities are approximately equal to 
the average intensities of radiation recorded at Cheshunt during the June 
and March experiments respectively. 
The proportion of infra-red radiation to total radiation was, however, 
greater than in sunlight, so that the intensities as they are stated above are 
