Betula Betula subsp. tortuosa

(c) Subsp. tortuosa View in CoL (Ledeb.) Nyman, Consp. 672 (1881)

(R. tortuosa Ledeb.; incl. B. kusmisschejfii (Regel) Suk. ):

Shrub up to 12 m, with many interlacing branches. Young twigs and leaves distinctly puberulent. Leaves less than 3 cm. Wing of fruit about as wide as nutlet. 2« = 56. Arctic Europe; mountains of Scandinavia; Iceland.

Plants very similar to those from the Arctic occur in Scotland and in the Alps, but require further investigation.

Hybrids between 1 and 2 have been recorded frequently, but cytotaxonomic investigation has shown that the first-generation triploid hybrid, which is highly sterile, occurs rarely, and that most of the putative hybrids have the chromosome number of the tetraploid species. It seems likely that such plants are the complex products of hybridization; they seem particularly common in regions such as Britain where most natural forest has been destroyed.

B. obscura A. Kotula , Jahresb. Schles. Ges. Vaterl. Kult. 65:314 (1888) (R. atrata Domin), from Poland, Czechoslovakia and W. Ukraine, is probably of such hybrid origin, resembling 1 in most characters except the dark-coloured bark. B. callosa Noto , Tromso Mus . Aarshefter 23 (1901), described originally from Norway, and reported from other parts of Fennoscandia and Iceland, differs from 2 principally in its ovoid, subsessile female catkins (cylindrical and pedunculate in 2) and the long, narrow lobes of the catkin-scales. Its relationships are not clear.

B. celtiberica Rothm. & Vase. , Bol. Soc. Brot. ser. 2, 14: 147 (1940), which replaces typical R. pubescens in the mountains of N. & C. Spain and N. Portugal, has glandular twigs, and bark which resembles that of 1. The form of the fruit and of the scale fall within the variation shown by 2 (a), and the chromosome number is the same (2n = 56). It seems best to treat itas conspecific with 2.