4 
COLOUR-REACTION. 
but after the application of a weak solution of iodine, the 
beautiful blue colour is developed, as shown in the figure. 
This colour is transient and would fade in a hour in daylight, 
but may be kept much longer in the dark. 
The lime and potash tests are generally used on the 
tliallus or leaf-like portion of the Lichen, sometimes on the 
surface only, and at others on the medullary layer. The 
results may be negative, producing no change in some species, 
while others will give different colour-reactions, thus affording 
much help in determining species. 
A glance at figure 2, plate 1, will show at once the strik¬ 
ing effect of an application of the lime test C. to the thallus of 
Roccella phycopsis, the portion A representing its natural 
colour, while B shows the red colour produced by a touch of 
this solution. It may further be noticed that the soredia 
(those white patches scattered over its surface) are not 
affected in this species by the C. solution, while in Roccella 
fuciformis , a species very near it, the thallus itself is 
untouched by an application of the C. solution, while the 
soredia are coloured red. It will now be clear how useful 
this test is in deciding to which of these species a specimen 
belongs. 
Leighton and some others not only use these tests in 
determining species, but also as a sufficient ground for form¬ 
ing new “varieties,” or even new species, as, to quote from 
Leighton’s Lichen Flora, “ Cladonia Florkeana var. bacillaris , 
K —, C —. Hitherto confused by external aspect and 
characters alone, with Cladonia macilenta , but separated by 
different reaction K yellow, C—.” 
While the classification of Lichens is generally based 
upon the spores, Leighton had found it more useful in 
Lecanora and Cladonia to make the colour-reaction of the 
thallus the basis of division, so that by carefully observing every 
effect of these colour-reagents a great help has been gained in 
the somewhat difficult study of the Lichens. 
It may not be generally known that the litmus test 
papers, to be bought in almost any chemist’s shop, are 
made from the Lichens of the genus Roccella. And it is 
probable that colour-reaction will before long form one of 
the standard tests of the biological laboratory. 
In the far greater number of my own trials the results 
have proved the accuracy of Leighton’s tests, and many 
times have I been able to name a Lichen which would have 
remained useless and undetermined but for the colour- 
reaction. 
