516 



PLANT GROWTH AND PLANT COMMUNITIES 



effect on its symmetry, the solitary, terminal, erect flower being usually 

 of radial symmetry even when it belongs to a family characterized by 

 zygomorphic flowers. Explanations of floral symmetry (Figure 17). 

 have tended to combine elements of both causality and teleology. 

 Space does not admit of an adequate treatment of this fascinating topic. 

 Goebel summarized his views by noting that if zygomorphic flowers 

 were entirely the result of variation in any direction, with selection 

 and survival of the fittest (i.e., those best adapted to insect visitors), 

 it is not evident why many terminal flowers should not also have be- 

 come zygomorphic; and- by noting also that dorsiventrality is found in 

 many anemophilous plants— e.g., many of the grasses. It will certainly 

 be interesting to see to what extent the several views on zygomorphy 

 accord with such results as may be obtained from critical morpho- 

 genetic investigations, undertaken without preconceived notions of 

 biological advantage, etc. That certain characteristic morphological 

 features of the adult flower may have an important selective advantage 

 in relation to insect visitation and cross-pollination may be accepted. 



Figure 17. Inflorescence of Digitalis purpurea; 

 its large, terminal, peloric flower is actino- 

 morphic, while the lateral flowers are zygo- 

 morphic. (After Velenovsky.) 



