This variety, which was long considered to be a distinct species, has been shown by Beissner and Schneider to be an abnormal form of B. pudescens. It differs from the type in the longer, more acuminate, slightly lobed leaves; and in the fruiting-catkins, which are very long (1½ inch or more) and slender, with pubescent ciliate scales, the middle lobe of which is elongated. The leaves and fruit are shown in Plate 269, Fig. 2.
This peculiar form has been found wild in the province of Wermland, in Sweden, and is only a tree of small dimensions. It is often planted in botanical gardens.
5. Some peculiar forms have arisen in cultivation, as var. aurea, young foliage tinted with yellow, sent out a few years ago by G. Paul, Cheshunt Nurseries; and var. nana, a dwarf form.
Hybrids
1. Hybrids have often been observed between this species and B. verrucosa, and have received various names, as B. hybrida, Bechstein, in Diana, i. 80 (1797); B. aurata, Borkhausen, Forstbot. i. 498 (1800); B. glutinosa, Wallroth, Sched. Crit. 497 (1822); and B. ambigua, Hampe, in Reichenbach, Fl. Sax. 120 (1842). These hybrids are intermediate in the characters of the branchlets, foliage, and fruit ; and may be suspected in cases where the branchlets are more or less glandular and show slight pubescence.
2. B. intermedia, Thomas, in Gaudichaud, Fl. Helvet. vi. 176 (1830), a hybrid between B. pubescens and B. nana, is a shrub about 1o feet high, with small leaves, which is found in the Swiss Jura, Greenland, Iceland, northern Europe, and Siberia. This has been collected’ in a few localities in the Highlands of Scotland.
Distribution
This species is widely distributed through Europe and northern Asia, extend- ing farther northward than B. verrucosa, but not descending so far south. It is the most northerly tree in Europe, growing on the shores of the Arctic Sea from North Cape (lat. 71°) to the mouth of the White Sea; its northern limit eastward through north-eastern Russia and Siberia to Kamtschatka being near the Arctic circle, though in some localities it ascends a degree or two higher. Its southern limit in Russia and Siberia appears to be the edge of the steppes, on which the tree does not grow; but it occurs in the Caucasus and Armenia. Farther westward the southern limit is the Carpathians and the Alps; and the tree is not found in the Apennines or in the Pyrenees, its most southerly point in France being near Grenoble. It is also a native of Iceland and of south-western Greenland. The distribution may then be roughly described as the northern hemisphere, from Greenland in the west to Kamtschatka in the east, between the parallels of 45° and 71°.
As compared with B. verrucosa, this species is found on the continent of Europe on wetter soils and in moister climates, and is the birch which grows on marshy ground and on undrained peat-mosses.
1 Cf. E.S. Marshall, in Journ, Bot. xxxix. 271 (1901), and Bot. Exchange Club Report for 1904, p. 33.
