THE EVOLUTION OF THE FLOWER 395 



monocotyledons, while four and five are common numbers in 

 the dicotyledons. A settling of the parts into fixed numbers 

 would be always an important forward step in floral evolution, 

 according to Engler, whether it concerns the perianth, the 

 sporophylls, or both together; for it tends to give definite form 

 to the flower, and thus leads toward the higher conditions. 

 Sometimes a flower will be mixed in the arrangement of its 

 parts, the perianth being cyclic and the stamens and carpels 

 spiral, as in certain buttercups. The establishment of fixed 

 numbers is frequently accompanied by the suppression of some 

 parts (sepals, petals, stamens, or carpels), so that the numbers 

 are variable in different circles. 



The apparent fusion of parts, frequently called coalescence, 

 results from the formation of tubular or cup-like outgrowths 

 from zones of tissue below the floral parts, so that they seem to 

 be united. The most complex conditions of flower structure, 

 called epigyny (Fig. 136, C) and perigyny (Fig. 136, B), are due 

 to these zonal growths (see Sees. 152, 157, 364). The contrast 

 to epigyny and perigyny is hypogyny (Fig. 136, A], When 

 petals or sepals are borne on zonal outgrowths the conditions 

 are called, respectively, sympetaly and synsepaly (Sec. 152). The 

 compound pistil, that is, a pistil involving two or more car- 

 pels, is one of the highest expressions of zonal growth ; this 

 condition is called syncarpy (meaning united fruits). 



The symmetry of the flower may be either radial or bilateral, 

 that is with a right and a left half (Sec. 150). Primitive flowers 

 were radially symmetrical, as would be expected from an in- 

 definite number of parts spirally arranged. Bilateral symmetry 

 appears, however, in very many groups and always represents a. 

 high degree of floral evolution. It is found more commonly in 

 epigynous and perigynous flowers than in hypogynous, but there 

 is no rule about its relations to these conditions. Bilateral sym- 

 metry is usually directly related to methods of flower pollination 

 by insects, for the forms of such flowers are especially adapted to 

 the habits of bees, which light on some expanded lip-like region 



