Part 122. The Birch Family - Various Plant Groups

Part 122. The birch family (Betulaceae). Examples: filbert (Fig. 23) and birch (Fig. 254).

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

We meet in this family with the singular form of inflorescence sometimes called "pussies," or catkins, and known botanically as aments.¹ 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 spicate² inflorescence, spike² 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 ³ 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 inflorescence of hazels (Corylus) the little heads (here two-flowered) are so few and crowded as to form a compound head of heads.⁴

¹ Am'ent - L. amentum, a thong or shoestring. Ij.

² Spi'cate, spike - L. spica, an ear of corn. L.

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, ].

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 surrounded by an involucel formed of its special bract and its two coalescent and adherent bractlets. Plants with both staminate and pistillate inflorescences borne upon the same individual plant are termed monoecious.⁵

⁵ Mo-nce'cious - Gr. monos, one; oikos, household. This is indicated by ♂-♀. If the staminate inflorescence differs in form from the pistillate the nature of each is shown by placing the inflorescence signs in corresponding order, i. e., beginning with the staminate. Thus Ii^:i" would read "staminate inflorescence amentaceous, the pistillate spicate, both compounded of heads."

The united bracts and bractlets of birches (Betula) ripen into dry scales forming a cone-like cluster of fruits made up of little samaras. In hazels the involucre becomes much enlarged in fruit, and each surrounds a much hardened pericarp which because of its hardness and indehiscence is called a nut.⁶

⁶ In the formula this extra hardness of the pericarp is indicated by two inverted exclamation points.

The family comprises woody plants without oil reservoirs but with resinous warts or hairs on the younger parts; simple, stipulate leaves; and monoecious inflorescences, the staminate amentaceous, the pistillate in spikes or heads with coalescent bracts and bractlets, and the pistils of two carpels with axile placentae.