THE ANGIOSPERMAE 1099 



important cause of zygomorphy is the unequal development or hetero- 

 morphy of parts. Any of the floral organs may be thus affected and the 

 examples are countless. To cite one conspicuous example where the petals 

 are heteromorphic although the ground plan of the flower is regular, we 

 see in Saxifraga sarmentosa (Fig. 1069) a flower in which two petals are 

 much larger and longer than the other three. A similar heteromorphy of 



Fig. 1069. — Zygomorphy of the corolla due to 

 heteropetaly in Saxifraga sarmentosa. 



the petals is very common in the peripheral flowers of crowded inflores- 

 cences, as in Umbelliferae, Compositae, etc., in which the outwardly directed 

 petals are much enlarged. The flowers of those families in which zygo- 

 morphy is most characteristic, 6'.^., Scrophulariaceae, Labiatae, Leguminosae, 

 show, however, combinations of more than one of the above features. The 

 more extreme the zygomorphy the greater is the number of the flower 

 parts which are affected by irregularities and the more marked are the 

 differences between them. 



The plane of symmetry in zygomorphic flowers is usually the antero- 

 posterior plane which passes through the bract and the inflorescence axis, 

 but in many Solanaceae such as Datura, the plane of symmetry is oblique, 

 due to the obliquity of the bicarpellary ovar}', and in a few cases, such as 

 Corydalis and Dicentra (in Fumariaceae), the plane of symmetry is trans- 

 verse. Obliquity of the plane of symmetry may also arise during floral 

 development through the torsion of the pedicel, as happens in Corydalis. 

 The complete inversion of the flower by the torsion of its support is called 

 resupination. It is met with consistently in the median zygomorphic 



