396 THE SPERMATOPHYTES 



of the perianth and rummage around, gathering pollen and 

 nectar, and incidentally effecting the pollination of the stigma 

 (Sec. 401). 



Bilateral symmetry is generally accompanied by dorsiven- 

 trality, which means that the flower hangs in such a position 

 that there is an upper and a lower portion as well as a right 

 and a left half. Excellent illustrations are such lipped flowers as 

 the snapdragons, the mints, and many orchids. An epigynous 

 flower whose symmetry is bilateral and dorsiventral and whose 

 parts, through suppression or other developments, show irregu- 

 larities which have a clear relation to insect visitations, these 

 characters give the highest types of flower evolution. 



According to Engler, the chief steps in the evolution of the 

 flower may be : 



1. The differentiation of a perianth. 



2. The change from spiral arrangement of parts, with indefi- 

 nite numbers, to cyclic arrangements, with fixed numbers. 



3. The grouping of parts through zonal growths (coalescence), 

 resulting in syncarpy, perigyny, and epigyny. 



4. The change from radial to bilateral symmetry, accompa- 

 nied by dorsiventrality. 



5. To these stages in floral evolution should be added the 

 complexity attained by the massing of numerous flowers in 

 groups or heads (Sec. 165), as in the composite family (daisies, 

 sunflowers, etc.). In the highest expressions of this development 

 the flowers are differentiated so that the outermost of the groups 

 become sterile, but by a remarkable lengthening of their corollas 

 into rays the flower cluster becomes very conspicuous. 



All flowers do not, by any means, follow the order of evolution 

 as outlined above, and there are very many special irregularities 

 in different groups. Thus certain flowers of the legume family 

 are bilaterally symmetrical and dorsiventral, but there is no 

 perigyny or epigyny. It is important to note that the higher 

 levels of flower evolution have been developed again and again in 

 unrelated groups of angiosperms independently one of another 



