THE BIRCH FAMILY 373 



leaves, and stame7is distinct and alternate, and the ovary with 

 but one cavity and one ovule. 



The formula of Polygonales is given on pages 412, 413. 



122. The birch family (Betulaceae) . Examples: filbert 

 (Fig. 23, page 3G) and birch (Fig. 254, page 265). 



See pages 412-415 for formulas of Betula, Corylus, and Betula- 

 ceae. 



We meet in this family with the singular form of inflores- 

 cence sometimes called "pussies," or catkins, and known 

 botanically as aments.^ An amentaceous inflorescence is 

 typicall}^ an elongated, often dangling, cluster of imperfect 

 flowers which are in the axils of scale-like bracts. It is a 

 special form of spicate ^ inflorescence, spike ^ being the 

 general term for a racemose cluster of sessile or nearly sessile 

 flowers. If the internodes of a spike fail to elongate the 

 flowers become crowded into a head or capitate ' inflorescence. 

 In the axil of each scale of a birch catkin we find three flowers 

 (Fig. 254) closely crowded together and so forming the simplest 

 sort of head. These heads of staminate flowers are borne 

 along the sides of a slender hanging rachis, so that the whole 

 compound cluster forms a typical ament. The pistillate 

 heads occur on a stiffer rachis which commonly grows erect, 

 and might therefore properly be called a spike although on 

 account of its scale-like bracts ])otanists often speak of this 

 inflorescence as a pistillate ament. In the pistillate inflores- 

 cence of hazels (Corylus) the little heads (here two-flowered) 

 are so few and crowded as to form a compound head of heads.* 



In the hazels the staminate flowers are solitary in the axils 

 of the scales, thus forming simple aments; while the pistillate 

 flowers are grouped in heads of two, and each flower is sur- 

 rounded b}' an involucel formed of its special bract and its 



^ Am'ent < L. amentum, a thong or shoestring. Ij. 



2 Spi'cate, spike < L. spica, an ear of corn. !=. 



^ Cap'i-tate < L. capitalus, having a head < caput, head. I". 



* All these facts are expressed in the formulas by using an inverted 

 exclamation point as the symbol of an amentaceous inflorescence, an 

 inverted colon for spicate, and two inverted periods for capitate clusters. 

 That the bractlets are adherent to the bracts by their lower parts is 

 shown by the small bracket, j. 



