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was observed: the indoxyl disappeared so completely from the leaves, 
that, after the said treatment, they became quite colourless, whilst 
pure hydrogen produced intensely blue leaves. 
In the carbonic-acid atmosphere there appeared, with the indigo, 
a small quantity of brown pigment, probably because the carbonic 
acid was not wholly free from air. The action of pure carbonic 
acid I have not yet examined. 
The vacuum in a barometer-tube, above mercury, gives the same 
result as the submersion in mercury itself, but this method is, of 
course, more complicated. 
3. On the „coloured strip” in partly killed leaves. 
The following phenomenon is in near relation to the preceding. In 
many Jeaves, when partly dying off, a coloured matter will appear, just on 
the border between the living and the dead tissue ; with woad and with 
Polygonum tinctorium, the chromogene of this coloured strip is indigo }), 
The experiment succeeds best if the leaf is partly killed by keeping 
it for a moment in the vapour of boiling water. The killed part remains 
green, although it may be a little more brownish than the living one. 
As for woad I think the phenomenon should be explained as 
follows. 0 
On the border between the dead and the living tissue, a strip of 
cells must occur which are in a condition of slowly dying. Accord- 
ing to the preceding description, alkali will be formed in these cells 
and the indoxyl quickly oxidises to indigo-blue, nothing of it 
finding time for disappearing in another way. If the partly killed 
woad-leaf, immediately after death sets in, is exposed to ammoniac- 
vapour, it becomes, as might be expected, over its whole extent 
deeply blue. If it is, before the action of the ammoniac, left for 
some time to the influence of the air, then some indoxyl gets lost 
from the killed part which colours with ammoniac, a little less 
strongly than what remained living. 
For Polygonum tinctorium the explanation is somewhat different 
because the indoxyl must first be originated by the action of the 
indigo-enzyme. But this enzyme is destroyed by the hot vapour 
in the quickly dying part, whilst on the border between the living 
and the dead part there must be a number of cells in which the 
1) With woad this experiment succeeds best with leaves from the rosettes of the 
first year in June; with Polygonum always equally well. In many other plants the 
vcoloured strip” does not contain indigo but a black or a brown pigment. 
ti 
