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A tree of small or medium size, occasionally 70 ft or more high; bark of trunk white, peeling off in papery layers, eventually dark and rugged at the base; young shoots downy, not warted. Leaves broadly ovate, 11⁄2 to 21⁄2 in. long, 1 to 2 in. wide; usually tapered, sometimes rounded or slightly heart-shaped at the base; pointed, coarsely toothed; upper surface thinly downy at first; lower one downy on the midrib and veins, sometimes only in the vein-axils, sometimes over the whole surface; veins in five to seven pairs; stalk more or less downy, 1⁄3 to 3⁄4 in. long. Fruiting catkins about 1 in. long; lobes of scale minutely downy, side ones rounded, terminal one ovate.
Native of Europe (including Britain) and N. Asia, and one of the two birches (the other is B. pendula) which make up the B. alba of Linnaeus. This is not so attractive a tree for the garden as B. pendula; its bark is darker, and its branching being more erect, it lacks the graceful, pendulous habit of that species. It affects moister places than B. pendula, and is especially abundant in Highland glens. Easily distinguished from typical B. pendula by its downy, not warted twigs.
It is a more variable species than B. pendula and in Flora Europaea is divided into three subspecies, of which two for certain occur in the British Isles:
subsp. pubescens. – This is the typical downy birch, which is usually treelike in habit, with downy young growths; leaves more than 11⁄4 in. long; wings of fruit as half as wide again as the nutlet.
subsp. carpatica (Willd.) Aschers. & Graebn. – Of more shrubby habit; young growths soon glabrous; leaves usually less than 11⁄4 in. long; wings of fruit as wide as the nutlet. Birches found in Scotland (referred to subsp. odorata in Flora of the British Isles) apparently belong here. Their young twigs and leaves are covered with resinous warts when young and are aromatic at that time; buds viscid. To this subspecies the Flora Europaea also refers var. murithii (Gaudin) Gremli, which is a dwarf tree up to 10 or 15 ft high, or a shrub, found in a few localities in Switzerland. It was discovered near Mauvoisin, Val de Bagne (Valais), and Dr Christ described it as a very pretty little tree. Var. pontica (Watson) Bean appears also to fall under this subspecies.
B. pubescens differs from B. pendula in having twice the number of chromosomes of that species (2n = 56). This difference restricts inter-breeding between the two species, and the first-generation crosses, when they occur, are sterile. The intermediates usually have the same chromosome number as B. pubescens, and must be hybrid swarms of complex origin (Flora Europaea),
The downy birch produces its seeds very freely, and is, as a rule, one of the first trees to find its way back to deforested areas. Like the grey birch in N. America, it is sometimes useful in affording shelter for young timber trees of better class. The wood is of very little value in this country except for turnery and pulping. In Scandinavia, where it is used for veneers, the occasional tree is found in which the wood is beautifully grained and commands a high price. A fragrant oil is obtained from it which is used in the manufacture of russia leather. The bark is water-tight, and is used in the construction of roofs in Sweden, etc. Under certain conditions it is curiously indestructible. I have seen pieces unearthed during peat-cutting in the Highlands, which must have been buried some centuries, but were still quite silvery.
The following are the most important garden varieties:
Another subspecies of B. pubescens is:
subsp. tortuosa (Ledeb.) Nyman B. tortuosa Ledeb. – A shrub or small bushy tree, often with a twisted stem, and with a dark bark. Leaves not much over 1 in. long, they and the branchlets densely downy. Native of Scandinavia and western Russia in high latitudes and in the mountains near the tree-line. Similar plants have been reported from Scotland and from the Alps of Switzerland and Austria.
† B. celtiberica Rothmaler & Vasconcellos – Twigs glandular as in B. pendula, but catkin-scales, nutlets and chromosome number as in B. pubescens. Bark white. A native of northern and central Spain and northern Portugal, described in 1940. The late Collingwood Ingram collected a seedling of this birch in 1929, in the Gerez mountains of Portugal, and planted it in his garden at Benenden, Kent, where it has made a handsome tree measuring 62 × 61⁄2 ft (1985); it has been propagated by grafting.
† B. litvinowii Doluhanov – This close ally of B. pubescens is a tree to about 50 ft high, with a white or pinkish peeling bark; young twigs glandular and downy. Leaves downy beneath on the veins when young, later almost glabrous, 1 to 2 in. long. Native mainly of the Caucasus and north-east Asiatic Turkey; described in 1939 and of recent introduction to Britain.
† B. ‘Fetisowii’. – The birch originally known by the horticultural name B. fetisowii was introduced by the Russian collector Fetisov towards the end of the last century from somewhere in central Asia. Schneider suggested B. humilis crossed with B. pubescens for a plant seen by him (but implying that other plants under the name were different). If, however, the cross occurred in the wild, a more likely parent is B. turkestanica, which is closely allied to B. pubescens. The plants known to Dippel were in fact received as B. alba turkestanica Fetisowii but these remained stunted and did not thrive.
Seed under the name B. fetisowii was received at Kew from the Pruhonice Arboretum, Czechoslovakia, in 1929, but the trees raised have been identified as B. pubescens and may be the result of pollination by that species at Pruhonice. Messrs Hillier received material under the name B. fetisowii from either Poland or Czechoslovakia at about the same time, but it is impossible to say whether these derive without adulteration from one of the original plants (which may themselves have been diverse). They have a chalky white bark but, as seen, poor, abnormally large leaves, from which no conclusions can be drawn.