I 20 
Gregory . — The Increase in Area of Leaves and 
not comparable with those in Table V. The radiant efficiency of the 
lamps used was taken to be 5 per cent., which indicates that the proportion 
of infra-red radiant energy to total energy is of the order of 95 per cent., 
while for sunlight at sea-level the same proportion is of the order of 50 per 
cent., for a sun at zenith in the latitude of Washington. The figures for 
intensity of incident radiation must therefore be divided by 10 to give 
values comparable with those in Table V, and will then give values of the 
same order as the average intensity of radiation for December. From the 
foregoing considerations it seems reasonable to suppose that the temperature 
of the leaves of the plants under the lamps was considerably higher than 
the air temperature, for the plants were in still air, and in an atmosphere 
75 per cent, saturated, both of which factors tend to check evaporation from 
the leaves. The air temperature was maintained at 35 0 C., and, moreover, 
the plants were exposed to this temperature during the whole period of 
growth. We may therefore, without much doubt, ascribe the action of the 
detrimental factor to the high temperature and low intensity of light radia- 
tion during these experiments. 
Relation of light intensity to ‘ average leaf are a\ In Table VIII the 
values for ‘average leaf area’ divided by total radiation for the Cheshunt 
set of experiments are stated. It is clear that for each set of experiments 
this value approaches constancy. We may therefore conclude that for 
comparable conditions the ‘ average leaf area ’ is determined by the product 
of intensity and of duration of light radiation. 
Table VIII. 
Total radiation 
Average 
Average leaf area. 
per sq . cm. 
leaf area. 
Total radiation. 
cals. 
sq. cms. 
Cheshunt — June . . 
7 ? o 77 
5 l6 
0.073 
March . 
2,470 
201 
0-081 
December 
849 
59 
0.069 
Under 2 lamps . 
. 8,086 
17.6 
0.0022 
1 lamp . . 
. 4,682 
10.2 
0*0022 
The part played by temperature in the increase of the leaf surface is 
not clear, and an investigation of this point is in hand. In the case of the 
Cheshunt experiments it appears that total radiation is the main controlling 
factor, while on the other hand the experiments with artificial light indicate 
that temperature relations may profoundly modify the action of light, if 
a super-optimal level is maintained. It would appear that for each light 
intensity there is a corresponding temperature at which the rate of increase 
of the total leaf surface attains a maximum, and if the temperature is raised 
above this level a detrimental factor comes into action tending to reduce the 
rate of increase of the leaf surface. 
The method of investigation described makes it possible to determine 
