always gave the same reaction, and that such tests assisted him greatly “in not only the discrimination of many difficult and closely allied species, but in some instances in defining the affinities of genera”. (Translation.)

For a time Tuckerman challenged these conclusions, but later acknowledged that chemical tests might yield “clews to affinity”. Prof. Bruce Fink also observed, “as to chemical tests, it is extremely doubtful whether they are of diagnostic value in any lichen”. Most modern students find chemical tests of very considerable value as supplementary evidence, but opinion is strongly divided on the validity of species distinguished solely on chemical differences from plants morphologically indistinguishable.

The writer deems all plants morphologically indistinguishable as members of one and the same species even when variations occur in the products of the internal metabolism. Some twenty distinct “lichen substances” are produced by the various species of Cladonia. Some of these are invariably present in certain species; some are variously present or absent. The former may be deemed diagnostic and the latter accessory; but the practice of separating out plants that possess or lack a “lichen substance” otherwise lacking or present as the case may be in a majority of specimens of a single morphological entity, and according them specific status does not seem warranted. Rather should they be deemed strains of one common species as has already been done in the Taxaceae, Myrtaceae, and Labiatae where plants from different localities have yielded different chemical compounds, or where the same plant has yielded different products at different seasons of the year.

The absence of a “lichen substance” normally present may be due entirely to environmental factors as when usnic acid fails to form in the absence of good sunlight; or it may be due to an errant gene, or to some other undiscovered cause. The fact remains that “species” segregated purely on chemical grounds have almost invariably the same geographic range as the parent species which is strong presumptive evidence that a single species is involved. The distinction between “accessory” and “diagnostic” lichen substances is often somewhat arbitrary, for it is the removal of the non-conformist plants that leaves any substance diagnostic for the balance. Where, however, a particular substance is known in but a single species (e.g., divaricatic acid in C. sullivani) then that substance can be regarded as truly diagnostic.

Notwithstanding what has been said above, the writer finds it expedient to give tentative recognition to the so-called chemical species in the flora, leaving to someone with a wider experience to make any relevant changes. The reagents most in use for species determination or verification are caustic potash (KOH), chloride of lime (CaCl₂O₂) and p-phenylene-diamine usually indicated by the letters K, C, and P followed by a + or − sign according as a colour change results or fails to occur. A faint reaction is indicated by + f. (KC) indicates that the application of K is immediately followed by C. For microchemical tests for the various lichen substances see Asahina (1).