394 THE SPERMATOPHYTES 



gymnosperms. Spiral arrangements of sporophylls (stamens and 

 carpels) and floral envelopes are also not uncommon in many 

 flowers with well-developed perianths, as in representatives of the 

 buttercup order, Ranunculales, namely, mousetail(Jf2/oswrws), but- 

 tercups, magnolia, white water lilies, and the rose order, Rosales. | 

 It is not at all probable that the various advances over the 

 primitive conditions followed any regular order. Some of them 

 were concerned with the differentiation of a perianth ; some had 

 to do with the arrangements of the sporophylls and parts of the 

 perianth; some dealt with the apparent fusion of parts, and 

 some concerned the symmetry of the flower. 



The differentiation of a perianth has clearly taken place in 

 some flowers through the transformation of sporophylls, which 

 became sterile and assumed perianth characters (generally those 

 of petals). Such transformations are admirably shown in the 

 passage of stamens into the parts of the perianth in the white 

 water lily, and in the doubling of flowers, where stamens and 

 frequently carpels become petals. It is possible, however, that 

 parts of a perianth may be derived in a reverse direction, that 

 is, from leaves or bracts on the stem just below the sporophylls. 

 That ordinary leaves can become highly modified and colored 

 to serve the purpose of a perianth is illustrated by the showy 

 bracts of the painted cup, or the flowering dogwood and other 

 species of Cornus (Frontispiece). The parts of the simplest types 

 of perianth were probably all similar and largely protective, 

 especially to the flower bud. These later became differentiated 

 into the two sets, sepals and petals, the latter, and frequently 

 also the former, showy and clearly related to pollination by 

 insects or birds (Chapter xxxn). 



The arrangements of the sporophylls and parts of the peri- 

 anth are, as a rule, spiral in simpler types of flowers, but gener- 

 ally in circles or whorls in higher types. In passing from the 

 spiral to the cyclic arrangements the variable and indefinite 

 numbers of parts tend to become constant. Thus three and 

 multiples of three are the prevailing numbers in the flowers of 



