While the ♀-flower exactly resembles that of the Alder, the reduction of the ♂-flower, already described in the Alder, is carried further, so that often only the 2 median perianth-leaves are developed (Fig. 328 _A_); there are also _only_ 2 stamens, these being deeply cleft, while the other 2 are suppressed. About 50 species; N. Temp.--Fossil-forms certainly occur in the Oligocene. During the Glacial period the Dwarf-birch (_B. nana_) extended over Europe; at the present time it is confined to the moors and mountains of N. Europe and N. America and Asia. Wind-pollinated. USES.--Important forest trees. The bark contains tannic acid. The tar of the Birch is used in the preparation of Russia leather; whilst its spring sap is very saccharine, and is used in some places for making a fermented drink. Its external bark is used for roofing, for baskets, etc. [Illustration: FIG. 327.--_Betula verrucosa_: _a_ cone-scale; _b_ fruit.] [Illustration: FIG. 328.--Diagrams of dichasia in the ♂-(_A_) and ♀-(_B_) catkins of Birch.] Order 2. =Corylaceæ= (=Hazel-nuts=). Monœcious. The ♂-catkins are long and cylindrical; the ♂-flowers are placed singly in the axil of the subtending-leaf (cover-scale); they are _naked_ and formed of a number of _divided_ stamens, which are partly united with the cover-scale, 4 in the Hazel, apparently 8 (Figs. 330 _A_, 329 _B_, _C_), more on the Hornbeam. The ♀-flowers have a very small, _superior_ perianth; in the axil of each cover-scale a 2-flowered dichasium (Fig. 329 _D_) is present, of which the central flower (* in Fig. 330 _B_) is suppressed. The gynœceum is bicarpellary as in the Birches; the ovary is bilocular, with two long styles (Fig. 329 _D-F_); the loculi have 1 (-2) ovules (Fig. 330 _B_). Each single ♀-flower and fruit is surrounded by a _leaf-like covering_, the _cupule_ (husk), which is _formed of three floral-leaves_ (namely, the bract of a lateral flower, and its own bracteoles; thus in Fig. 330 _B_, α, α′, β’ form the cupule for the left-hand flower, and β, α_[1}, β_[1}, the cupule for the right-hand).