222 



NATURAL HISTORY OF PLANTS. 



have alternate simple leaves, dentate or entire, not persistent, with 

 petioles accompanied at the base by two lateral caducous stipules. 

 Young, they are plicate and equitant in the interior of a scaly bud. 



The flowers are generally monoe- 

 cious and collected in unisexual 

 catkins, which are solitary, or 

 more rarely in clusters, 1 to the 

 number of two or four as in the 

 Asiatic species constituting the 

 genus Betulaster. 2 In the axil 

 of each scale of the male catkin, 

 there is a cyme, formed generally 



BtUda alba. 



"ir 



Fig. 154. Male floriferous 

 scale without flowers. 



rig. 153. Male flowers. 



of three flowers, a median and 

 two lateral, rising from the axil- 

 lant scale and accompanied by 

 two secondary scales, similarly 

 supported and interior, one on 

 each side. 3 In the female cat- 

 kins, there is in the axil of each 

 scale, accompanied also by four 

 secondary scales, a biparous cyme 

 three- or more-flowered, often re- 

 duced to two flowers. 4 In the 

 fructiferous catkin, the principal 

 accrescent scales accompanied by 

 the secondary scales embodied 

 with them, 5 are detached early 

 or persist for a longer or shorter 

 period on the axis of the catkin, 

 with the samaras, which they completely conceal in all the Birches 



Fig. 156. Triflorous fe- 

 male cyme. 



Fig. 151. Young foli- Fig. 157. Long. sect, of 

 aceous branch. female flower \. 



1 Often, as in B. fruticosa, the axis of a fe- 

 male catkin thickens and its lower portion 

 persists and ultimately developes into a branch 

 which, the following year, bears leaves and 

 flowers, the female catkins of which will like- 

 wise have a persistent base. 



2 Spach, Ann. Sc. Nat. sér. 2, xv. 182, 198.— 

 Endl. Gen. Suppl. iv. p. ii. 20. 



3 They have often been considered as sti- 



pules of the principal bract or scale. Previous 

 to their late displacement they appear, from the 

 sit n.it ion, to represent two lateral bracteoles, the 

 axil of which would be occupied by the lateral 

 flowers of the inflorescence. 



4 From abortion of the terminal flower, not 

 unfrequent in this genus. 



5 So that the whole then appears a rigid 

 bract, trilobed above. 



