524 DR LAUDER LINDSAY ON NEW LICHENICOLOUS MICRO-FUNGI. 



ordinary form. There is a substance in lichens that gives a beautiful and deep blue 

 reaction with iodine ; and if chemists are correct in asserting that such a reaction 

 is indicative of the presence of free starch in its ordinary form, while lichenine 

 and inuline give yellow or brown colours to the reagent, we must admit that 

 common starch not only occurs in the lichen-tissues, but that it is sometimes 

 associated with, and at other times substituted for, lichenine and inuline. 



II. Gum, or its modifications. 



Gum has been examined as it exists in Lecanora parella Gives a greenish- 

 blue with iodine (Schunck). Ordinary gum (= Arabin) is not altered in colour 

 by iodine; but the modification thereof known as Bassorin gives blue and red 

 reactions (Miller, p. 109). 



Were we to accept as a trustworthy basis for our conclusions the foregoing 

 assertions of chemists, we would deduce that in lichens occur several forms or 

 modifications of starch and gum that give reactions with iodine, variously blue, 

 red, or brown, or admixtures of these shades, especially green. But it is impos- 

 sible to accept as proper bases, on which to found diagnostic characters, state- 

 ments so contradictory. The conclusion to be drawn is rather that chemists are 

 yet ignorant in great measure of the composition and character of the muci- 

 laginous and other components of lichens ; and that at present they probably 

 confound substances of somewhat dissimilar character. Thus the character of 

 the iodine-reaction leads to the suspicion that what Schunck describes as a gum, 

 may be in reality a starch ! 



It by no means follows that the same reagent should produce the same colour- 

 reaction in the same species of lichen, whether it is applied by the chemist in 

 the laboratory to the separated amylaceous or mucilaginous principles, or by the 

 lichenologist in his library to microscopical sections or preparations of the 

 hymenium or other tissues. On the contrary, what we know of other colour- 

 developments in lichens would lead us here to expect a certain difference in 

 result ; and, in point of fact, there is such a difference. And, further, differences 

 of result in the same species, when iodine is applied as a test in microscopico- 

 botanical diagnosis, arise in the hands of different experimenters from circum- 

 stances sometimes apparently most trivial, e.g., the strength or character of the 

 iodine solution, the age or other conditions of development of the specimen 

 operated on. I need not, however, further pursue or illustrate the subject here, 

 having pointed out in detail elsewhere the sources of fallacy and the causes of 

 difference in the colour-reactions of lichens as supposed botanical characters.* 



The substances or tissues in lichens, which yield colour-reactions with iodine, 

 are chiefly — (1.) The hymenial gelatine or mucilage, Avhich has hitherto been 



* " On Chemical Reaction as a Specific Character in Lichens," Journ. of Linn. Soc. vol. xi. 

 (Botany), p. 36 ; and " Experiments on Colour-reaction as a Specific Character in Lichens," Trans. 

 Botanical Society of Edinburgh, vol. x. 



