126 
Proceedings of Royal Society of Edinburgh. [mar. 7, 
[Trans.'], 1883, 1884, 1885, 1886, 1887). In the experiments to 
be detailed here, we have used ferrous sulphate as an indicator of 
root-absorption. 
In seven small flower-pots, each filled with the same kind of soil, 
and treated with a known weight of ferrous sulphate, were sown 
some mustard seeds. When the little plants had made their appear- 
ance above the soil, they were exposed for some hours each day to 
the coloured lights of the spectrum. 
No. 1 was exposed to the red part of the spectrum ; No. 2 to 
the orange, and so on ; after so many hours’ exposure they were 
removed to a dark place. This operation was performed for several 
weeks, each pot being exposed to its own part of the spectrum daily 
until the plants had grown to a considerable size. The plants in 
each pot were then reduced to ashes, and submitted to analysis, the 
following percentage of ferric oxide being found : — 
Mustard Plants. 
Pot No. 1 exposed to red part of spectrum, gave 
,, 2 ,, orange 
,, 3 ,, yellow 
„ 4 ,, green 
,, 5 ,, blue ,, 
„ 6 ,, indigo 
., 7 ,, violet „ 
Percentage of Fe 2 0 3 
in Ash. 
. 0-92 
. 1-43 
. 2-51 
. 1-90 
. 071 
. 0-20 
. 075 
The same number of seeds were placed in each pot, and each pot 
received the same quantity of iron sulphate during the investigation. 
The plants, as they grew, were watered from time to time with a 
weak solution of ferrous sulphate, always of the same strength (see 
figure). 
We have also tried the same experiments upon bean seeds (work- 
ing exactly under similar conditions as those detailed above), with 
the following results : — 
Bean Plants. 
Percentage of Fe 2 0 3 
in Ash. 
Pot No. 
] exposed to red part of spectrum, gave 
. 1-40 
9 9 
2 
99 
orange 
99 * 
. 275 
n 
3 
9 9 
yellow 
9 9 
. 4-52 
9 9 
4 
9 9 
green 
9 9 
. 3-34 
9 > 
5 
9 » 
blue 
99 * 
. 172 
9 9 
6 
9 9 
indigo 
9 9 
. 0 84 
9 9 
7 
99 
violet 
9 9 
. 0-53 
