176 The Phenomena of Morphogenesis 



in which the flowers are borne laterally on an inflorescence, this radial 

 symmetry has become dorsiventral and the flower is said to be zygomor- 

 phic (irregular; Fig. 7-14). The pea flower, with its standard, two wings, 

 and keel is a familiar example, and the median plane of symmetry here 

 is especially well marked. Flowers of this sort provide many of the 

 notable adaptations for insect pollination. In most cases zygomorphy is 



Fig. 7-14. Dorsiventrally symmetrical ( zygomorphic ) flower of Linaria vulgaris. 

 ( Courtesy of Rutherford Piatt. ) 



evident from the beginning of development and is unaffected by the rela- 

 tion of the flower to gravity or other environmental factors. In other 

 cases (such as Epilobium, Friesia, and Digitalis), if the flower develops 

 in a vertical orientation or on a clinostat, it becomes radial, indicating 

 that dorsiventrality here is directly affected by gravity (Fig. 7-15). In 

 cases of peloria (p. 282) the flower of a species which is normally zygomor- 

 phic (as in Linaria or Digitalis) may become radially symmetrical. Most 



