172 LICHENOLOGICAL MEMORABILIA. 



Lecidea Dilleniana, (Ach.), and lias been often confounded with it 

 in the published continental " Exsiccati." Attention, however, to 

 the chemical reaction of the tballus and the size and form of the 

 spores enables us distinctly to distinguish the two plants. In L. 

 Dilleniana the reaction of the thallus is K yellow, C orange-yellow, 

 and the spores are large, aciculari- fusiform, acute at the apices, and 

 3-septate (see fig b.). In 0. grvmidosa, on the contrary, the re- 

 action of the thallus is K very faint yellow, C decided red, and the 

 spores less than half the size of those of L. Dilleniana, oblongo- 

 fusiform or lineari-fusiform, obtuse or rounded at the apices, 

 3-septate (see fig. d.). 



When describing L. Dilleniana in my " Lichen- Flora of Great 

 Britain," &c, p. 332, I had never seen or examined a specimen of 

 0. grumulosa, which at that time (1871) had never been detected 

 as a British lichen, except in Jersey, by Mr. Larbalestier. Conse- 

 quently whilst giving in the Lichen-Flora the proper reaction to 

 L. Dilleniana, I mentioned in a note that the " Exsiccati " of 

 Anzi Ven, 82 and 83, and Zwackh, 142 (respectively labelled as 

 L. Dilleniana), and Schasr., 580 (labelled as Lecidea epipolia), 

 had a different reaction, viz., K yellow, C red. The present exami- 

 nation of authentic specimens of 0. grinnulosa now enables me to 

 refer these to their proper place, viz., to 0. grumulosa, and not to 

 L. Dilleniana. 



Much has been written time after time by various writers against 

 chemical tests and their supposed variability, and this apparent 

 diversity has no doubt resulted from similar circumstances with the 

 above. But the above fact appears to my mind as an additional 

 confirmation of their real value and constancy. In these investi- 

 gations it will be advisable to bear constantly in mind the caution 

 — a caution which cannot be too often repeated — which Dr. 

 Nylander gives in " Journ. Linn. Soc," ix., p. 365, note : — " The 

 analyses of lichens made by chemists often fail through the neglect 

 of an exact determination of the species, and probably not less 

 often by the mixture of specimens confounded together, and in- 

 correctly assigned to one single species. For the chemist, no less 

 than for the physiologist, it is of the greatest importance to know 

 exactly what is the plant we have under observation ; that is, to 

 have well determined the plant which we are studying." In other 

 words, he must not place implicit confidence on the attached labels 

 merely, from however great soever authority resulting, as indicating 

 undoubted accuracy, nor on his own preconceived notions of the 

 particular diagnosis ; but by careful observation and comparison 

 thoroughly satisfy himself that the plant under review is really 

 that which the label indicates it to be. Then apply the chemical 

 test, and doubt will be exchanged for certainty. 



It affords no trifling pleasure to my own mind to acknowledge 

 and correct the error into which, through ignorance and imperfect 

 knowledge, I had fallen, and at the same time to adduce a confirm a- 



