>54 



MORI'IIOLOCN 



broadens, so that the final stage is one in which the succession of cycles 

 seems to be centripetal rather than acropetal. 



Associated with the appearance of the cyclic stage is the establishment 

 of a definite number of organs for each cycle. For example, the definite 

 floral number for cyclic monocotyledons is three, but there are many 

 spiral monocotyledons with no definite number ; the definite floral 

 number for cyclic dicotyledons is usually five or four, but there is a host 

 of spiral dicotyledons in which the numbers are indefinite. The cyclic 

 condition is not necessarily attained simultaneously by all the regions of 



Figs. 566-568. — Pistils: 566, apocarpous; 567, syncarpous with free styles; 568, com- 

 pletely syncarpous. — After Berg and Schmidt. 



the flower. For example, in the buttercup the sepals and petals usually 

 show the definite cyclic number five, but the stamens and carpels are 

 still in the spiral condition of indefinite numbers (tig. 562). 



Zonal development. — Another evident tendency among the flowers 

 of angiosperms is the so-called coalescence of members of the same set. 

 For example, the zone of tissue upon a receptacle which is giving rise 

 at several points to a cycle of separate petals, sooner or later begins to 

 develop uniformly, resulting in a corolla tube instead of several petals 

 (fig. 563). This is called zonal development, and the sooner it begins the 

 more completely tubular does the corolla become. This tendency to 

 zonal development is observed in all the floral cycles, and this condition 



