222 



PLANT STRUCTURES 



" without petals." It is not always possible to tell whether 

 a flower is apetalous — that is, whether it has lost a floral 

 set which it once had — or is simply one whose perianth has 

 not yet differentiated, in which case it would be a "primi- 

 tive type." 



The line of evolution, therefore, extends from flowers 

 without floral leaves, or naked flowers^ to those with a dis- 

 tinctly differentiated calyx and corolla. 



121. Spiral to cyclic flowers. — In the simplest flowers the 

 sporophylls and floral leaves (if any) are distributed about 

 an elongated axis in a spiral, like a succession of leaves. 

 That part of the axis which bears the floral organs is for 

 convenience called the receptacle (Fig. 202). As the recep- 



FiG. 202. A buttercup {Ranunculus): a, complete flower, showing sepals, petals, sta- 

 mens, and head of numerous carpels on a large receptacle; b, section showing 

 relation of parts; a hypogynous, polypetalous, apocarpous, actinomorphic flower. 

 —After Baillon. 



tacle is elongated and capable of continued growth, an in- 

 definite number of each floral organ may appear, especially 

 of the sporophylls. With the spiral arrangement, there- 

 fore, there is no definiteness in the number of floral organs ; 

 there may be one or very many floral leaves, or stamens, or 

 carpels. The spiral arrangement and indefinite numbers 

 are features of the ordinary strobilus, and therefore such 

 flowers are regarded as more primitive than the others. 



In higher forms the receptacle becomes shorter, the 

 spiral more closely coiled, until finally the sets of organs 



