33 
THE BRITISH SPECIES OF THYMUS. 
By K. Domin & A. Bruce Jackson. 
Thymus is one of the many genera which present special diffi- 
culties to the systematist from the fact that it includes a large 
pee of critical forms which are very hard to define. This is 
roc 
the steppes of North Bohemia being the northernmost limit of 
these South-east Euro species. Central and North Europe . 
and the Alps ~ id a few species, but Nets are represented by 
an extraordina mber of variable form 
Until receely” the characters aiinlovadl for differentiating the 
species and varieties of Thymus were of doubtful value, and many 
plants which have been given specific rank can hardly be con- 
sidered more than forms, or at the most varieties. Moreover, it 
would be necessary if these Poe tart based upon the size and 
colour of the flowers, the degree of pubescence of the calyx and 
stems, the shape of rs ue the smell, the more or less elon- 
gated branches, &e., were used as distinctions to describe new 
species ad infinitum, bria the manner of Opiz and Déséglise. 
This ie render an already complicated synonymy still more 
cumberso 
Boeke: who published a revision of Thymus (“ Symbole ad 
hymos Europe Medie praecipue Hungarie oscendos,” 
Math. és Természet Kézlemények Kértet, xxiv. pp. 39-116, Buda- 
pest, 1890), which treats especially of the Central and South-east 
European forms, laid stress upon charac erived from the 
nervation of the leaves and the me of the stems, which at 
the —. are artificial and etter 
! ed a more natural 
grouping of the plants than had been adopted by previous authors. 
raed is redone from his “ Vorstudien zu einer Monographie der 
g ” Beth. z. Bot. Centralbl., Berlin (1906). Borbis, 
sist especially Velondvak¥: show us how important a part hybri ridi- 
zation plays in Thymus, an min has found that wherever two 
corpadaily the result of crossing; as is the case with many species 
of Potentilla and some other gene 
It is the occurrence of such ybeids which renders the study 
of this genus an unusually difficult one. Dried specimens, often 
very imperfect, because failing to show the mode of branching, are 
frequently not determinable. It is absolutely essential to have 
complete plants to be quite certain to which species they belong. 
Pe itie! OF Borany.—Vot. 46. [FEBRUARY, 1908. } D 
