Fig. 325.—Alnus glutinosa: A dichasium of ♂-flowers seen from the front; B the same from inside; C the same from the back; D dichasium of ♀-flowers with subtending-leaf and four bracteoles. The letters b, α, β, β′, β are the same as in Fig. [326] A.
Fig. 326.—Alnus glutinosa: diagram of dichasia of ♂ (A) and ♀ (C) catkins; B a cone-scale. All the bracteoles in A and C are slightly pressed from their normal position.
The Inflorescences of the Alder.—In the axil of each cover-scale [b in the Figs] is situated, in the ♂-catkins (Figs. [326] A, [325] A-C) a 3-flowered dichasium, the flowers of which have a 4-partite perianth, the posterior perianth-segments being sometimes almost suppressed, and 4 stamens with undivided filaments. In the ♀-catkin (Figs. [325] D, [326] C) a 2-flowered dichasium is found, the middle flower being suppressed (indicated by a star in C). In both instances the inflorescences have two bracteoles (α-β) and the flowers borne in their axils have each one bracteole (β′), the other one (α′) being suppressed and therefore in 326 A and C only represented by a dotted line; these four bracteoles unite with the cover-scale (b) which supports the entire dichasium, to form the 5-lobed “cone-scale” (Fig. [326] B) which in the ♀-catkin eventually becomes woody.
The Inflorescences of the Birch.—A 3-flowered dichasium is situated in the axil of the cover-scale in both ♂-and ♀-catkins (Fig. [328] A, B); only the central flower has bracteoles (α-β) (the lateral flowers having no bracteoles), and these bracteoles unite, as in the Alder, with the supporting cover-scale (b), and form a three-lobed cone-scale (Fig. [327] a).
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.
