
p. 
, 
} 
: 

1905.]. On Vegetable Assimilation and Respiration. 403 
Particularly is this the case when natural illumination is being investigated, 
for the diffuse light of the sky produces but little heating effect, while direct 
solar radiation may heat up a leaf considerably. 
During the summer of 1904 we have been working at the relation between 
carbon-dioxide assimilation and the intensity of natural illumination. For 
this work we determined the real internal temperatures of our assimilating 
leaves by the thermo-electrical method described in “ Assim. and Resp. III,” 
p. 76. Without the exact data as to the relation between temperature and 
assimilation set out in that paper, and without the knowledge gained when 
working with artificial light, we should have been quite baffled in our attempt 
to deal in detail with all variations of natural light. 
In several directions the present paper is of a “ preliminary” nature, and 
we hope to complete the work next summer. 
We determined, at the outset, to work in the open air, so as to be able to 
use direct sunlight, and to avoid the use of heliostats and reflecting silver 
surfaces.* 
The general experimental and analytical procedure has been the same as in 
“ Assim. and Resp. III.” 
As it was, however, impossible to work sensitive aspirators and a galvano- 
meter satisfactorily when they were exposed to direct or intermittent 
sunshine, these parts of the apparatus were set up in a north room of the 
laboratory. Thence they were connected by 50 feet of tubing and wires 
to the leaf chamber, and to the baryta-absorption-tubes situated on a table 
on the flat roof of the new University Botany School. On this spot sunshine, 
when vouchsafed, is continuously available, without any interfering shadows, 
for quite 12 hours daily, in the height of summer. 
In addition, gas for heating, and running water for cooling are there 
conveniently to hand, so that the temperature of the leaf in its chamber can 
be fully controlled. Without all these facilities, such as are afforded by very 
few botanical laboratories, our particular work would have been impossible. 
The leaves employed were those of cherry-laurel and of Helvanthus 
tuberosus. With the former we have worked for some years, and the latter 
were employed in order to test whether such a different type of leaf 
would give similar results. 
The cut leaf is set up in the usual flat leaf-chamber with a fixed glass 
front and an adjustable glass back, through which pass the wires from the 
thermo-junction in the leaf to the galvanometer. 
* Langley (‘ Phil. Mag.,’ 1889, p. 10) has shown that the various solar rays are reflected 
in slightly different proportions by polished silver, and also that the least tarnishing is a 
serious disturbing factor. . 
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