GASTANEACE^. 



221 



the two branches of a filament which, simple at its base, bifurcates 

 at a variable height like the letter Y\ The female flowers are 

 destitute of perianth and composed only of a free gynoecium with a 

 bilocular ovary,^ and surmounted by a style almost immediately 

 divided into two long subulate branches, covered with stigmatic 



Betula pnmila. 



Fig. 147. Triflorous male Fig. 148. Long. sect, of Fig. 149. Male 

 scale (f). triflorous male scale. flower. 



Fig. 150. Fruit (4). 



Betula alba. 



papilUie. In each of the ovarian cells (which are, like the styles, 

 anterior and posterior), there is, in the internal angle, a placenta 

 supporting a single^ descending anatropous ovule, with the micropyle 

 directed upwards and outwards.* The 

 fruit, flat and edged with two mem- 

 branous wings perceptible on the ovary 

 and rendering it samaroid, is dry^ and 

 indehiscent, interlocular and monosper- 

 mous by abortion of one of its seeds, ^ 

 whilst the other is fertile and encloses 

 under its coats a fleshy embryo, straight 

 and destitute of albumen, with superior 

 radicle and cotyledons fleshy and nearly 

 flat. — The Birches are trees and shrubs 

 growing in the cold and temperate regions of both worlds."^ They 



Fig. 152. Male 

 catkins. 



Fie 



, 155. Female 

 catkin. 



' Admitting four stamens they have, in con- 

 sequence, been described as diadelphous. 



2 In reality unilocular and possessing pri- 

 marily two parietal placentse which unite near 

 the centre of the cavity, one of them generally 

 becoming more or less completely abortive. 



3 Very rarely two ovules correspond to one 

 cell, only one of which is perfectly developed. 



* It has a simple coat. 



' At the centre, the ovary is traversed by a 



vertical fascicle, itself surrounded by a disunited 

 cellular tissue, forming part of the partition, 

 very thick below, of the pericarp. 



^ Frequently there are two, but in that case 

 one or the other is often sterile. 



7 As are the Betula in general, except one 

 species of Alnus which inhabits southern 

 Africa. (Reg.) Those which, in much smaller 

 number, are observed in tropical Asia and 

 America, grow on high mountains. 



