QUERCIFLORJ;. 343 



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 (IS. 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 ex- 

 ternal bark is used for roofing, for baskets, etc. 



PIG. 327. Betula tiernieosa : a cone-scale ; PIG. 328. Diagrams of dichasia in 



b fruit. the cj- (-4) and ?-(#) catkius of Birch. 



Order 2. Corylacese (Hazel-nuts). Monoecious. 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 JB, G), 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 gynoeceum 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, a, a', ft' form the cupule for the left-hand 

 flower, and j3, a ; , /? the cupule for the right-hand). 



Gunjlus (Hazel-nut, Fig. 329). The long, cylindrical ^-catkins 

 pass the winter naked, 2-3 together, on short branches. The very 



