268 PLANT STRUCTUKES 



Sympetala 



143. Introductory. These are the highest and the most 

 recent Dicotyledons. While they contain numerous shrubs 

 and trees in the tropics, they are by no means such a shrub 

 and tree group in the temperate regions as are the Archi- 

 chlamydeae. The flowers are constantly cyclic, the num- 

 ber five or four is established, and the corolla is sympeta- 

 lous, the stamens usually being borne upon its tube (Figs. 

 208, 209, 212). 



There are two well-defined groups of Sympetalse, distin- 

 guished from one another by the number of cycles and the 

 number of carpels in the flower. The group containing 

 the lower forms is pentacyclic, meaning " cycles five," there 

 being two sets of stamens. In it also there are five carpels, 

 the floral formula being, Sepals 5, Petals 5, Stamens 5 + 5, 

 Carpels 5. As the carpels are the same in number as the 

 other parts, the flowers are called isocarpic, meaning " car- 

 pels same." The group is named either Pentacyclce or Iso- 

 carpcB, and contains about ten families and 4,000 species. 



The higher groups, containing about forty families and 

 36,000 species, is tetr acyclic, meaning " cycles four," and 

 anisocarpic, meaning "carpels not the same," the floral 

 formula being, Sepals 5, Petals 5, Stamens 5, Carpels 2. 

 The group name, therefore, is Tetracyclw or Anisocarpce. 



144. Heaths. The Heath family (Ericaceae) and its allies 

 represent about two thousand species. They are mostly 

 shrubs, sometimes trailing, and are displayed chiefly in 

 temperate and arctic or alpine regions, in cold and damp 

 or dry places, often being prominent vegetation in bogs 

 and heaths, to which latter they give name (Fig. 254). The 

 flowers are pentacyclic and isocarpic, as well as mostly hyp- 

 ogynous and actinomorphic. It is interesting to note that 

 some forms are not sympetalous, the petals being distinct, 

 showing a close relationship to the Archichlamydese. One 

 of the marked characteristics of the group is the dehiscence 



