1762 



A TEXTBOOK OF THEORETICAL BOTANY 



been discovered in Cretaceous and Tertiary rocks showing that at that time 

 the northern hmit was extended to Greenland, Spitsbergen and Iceland. 



Betulaceae 



Several common British trees are included in this family. Betula 

 verrucosa (Silver Birch), Alniis glutincsa (Alder), Carpimis betulus 

 (Hornbeam), Corylus avellana (Hazel) are the best known. 



The plants are either trees or shrubs, with alternate leaves bearing 

 monoecious flowers in unisexual, catkin-like, compound inflorescences. 

 The inflorescence in the male is a catkin which terminates the growth 



of a branch in Betula, but in Corylus it is 

 borne on a dwarf shoot. The bracts are 

 arranged spirally and in the male the 

 flowers are united to the bracts and brac- 

 teoles. In the female the catkins terminate 

 leafy shoots and each consists of spirally 

 arranged bracts in the axil of which is a 

 dichasium of three flowers, the centre one 

 of which is usually missing. Hence the 

 inflorescence is really compound. 



The perianth (Fig. 1644) when present 

 consists of small scale leaves which vary in 

 number and may be either free or united 

 together. In the female flowers the ovary is 

 inferior; the male flowers adhere to their 

 bracts. 



The androecium consists of from two to twelve stamens (Fig. 1645) 

 each of which is divided nearlv to the base. 



Fig. 1644. — Floral diagrams of 

 Corylus avellona. Betulaceae. 

 Female flower above ; male 

 flower below. {After Eichler.) 



BcJ 



Fig. 1645. — Betula. Floral structure. A, Female flowers in 

 cyme of three, each consisting of two joined carpels with 

 two styles. B, Cyme of male flowers. 



The gynoecium consists of a bilocular ovary v.ith two styles. Each 

 loculus contains one pendulous anatropous ovule with a single integument. 



