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Salix alba
''Salix alba'' (white willow) is a species of willow native to Europe and western and central Asia.〔Meikle, R. D. (1984). ''Willows and Poplars of Great Britain and Ireland''. BSBI Handbook No. 4. ISBN 0-901158-07-0.〕〔Rushforth, K. (1999). ''Trees of Britain and Europe''. Collins ISBN 0-00-220013-9.〕 The name derives from the white tone to the undersides of the leaves.
It is a medium-sized to large deciduous tree growing up to 10–30 m tall, with a trunk up to 1 m diameter and an irregular, often-leaning crown. The bark is grey-brown, and deeply fissured in older trees. The shoots in the typical species are grey-brown to green-brown. The leaves are paler than most other willows, due to a covering of very fine, silky white hairs, in particular on the underside; they are 5–10 cm long and 0.5–1.5 cm wide. The flowers are produced in catkins in early spring, and pollinated by insects. It is dioecious, with male and female catkins on separate trees; the male catkins are 4–5 cm long, the female catkins 3–4 cm long at pollination, lengthening as the fruit matures. When mature in midsummer, the female catkins comprise numerous small (4 mm) capsules, each containing numerous minute seeds embedded in white down, which aids wind dispersal.〔〔〔Mitchell, A. F. (1974). ''A Field Guide to the Trees of Britain and Northern Europe''. Collins ISBN 0-00-212035-6〕
==Ecology==
White willows are fast-growing, but relatively short-lived, being susceptible to several diseases, including watermark disease caused by the bacterium ''Brenneria salicis'' (named because of the characteristic 'watermark' staining in the wood; syn. ''Erwinia salicis'') and willow anthracnose, caused by the fungus ''Marssonina salicicola''. These diseases can be a serious problem on trees grown for timber or ornament.
It readily forms natural hybrids with crack willow ''Salix fragilis'', the hybrid being named ''Salix × rubens'' Schrank.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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