THE BIRCH FAMILY 378 
leaves, and stamens 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 (Betulacez). Examples: filbert 
(Fig. 23, page 36) and birch (Fig. 254, page 265). 
See pages 412-415 for formulas of Betula, Corylus, and Betula- 
cee. 
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 
typically 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 botanists 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 by an involucel formed of its special bract and its 
1 Am’ent < L. amentum, a thong or shoestring. Jj. 
2 Spi’cate, spike < L. spica, an ear of corn. I:. 
3 Cap’i-tate < L. capitatus, having a head < caput, head. I’. 
4 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, 3. 
