OEGAXS OF EEPEODrCTIOK. 349 



dimerous, and the symmetry is said to be Fig. 768. 



binary or two-memhered : this may be con- 

 sidered as answering to the distichous or 

 two-ranked arrangement of leaves (see page 

 137) ; each whorl forming one cycle com- 

 posed of two parts the intemodes between 

 the several parts not being developed, or 

 to successive pairs of opposite leaves de- 

 cussating with each other. This arrange- 

 ment is thus marked, ^/. When there are "^^^'flower^'f ^c^™ °^ *^^ 

 three parts in a whorl, as in the Squill °^^^ ° "''^^'^ 

 {fig. 422), Iris, and Lily, the flower is trimerous, and the 

 symmetry is ternary, trigonal, or triangular-, it is indicated 

 thus, ^, This may be regarded, either as answering to the 

 tristichous arrangement of leaves, each whorl forming a cycle of 

 three organs, the internodes between them not being developed; 

 or to successive whorls of three organs in each. When there are 

 four parts in a whorl, as frequently in Rue {fig. 564), the flower 

 is tetramerous, and the symmetry, which is marked y, is quater- 

 nary, tetragonal, or square. The successive whorls in such a 

 flower may be compared directly with whorls of leaves, each 

 consisting of four organs ; or indirectly with opposite decussat- 

 ing leaves combined in pairs, the intemodes not being de- 

 veloped. When there are five parts in a whorl, as in Crassula 

 rubens {fig. 766), the flower is said to he pentamerous, and the 

 symmetry, which is marked thus, ^/, quinary or pentagonal. 

 Such a flower may be considered as answering to the penta- 

 stichous arrangement of leaves with the internodes undeveloped; 

 or to be composed of successive whorls of five leaves, the inter- 

 nodes between each whorl being almost undeveloped, or very short. 



Of the above arrangements, the pentamerous is most common 

 among Dicotyledons, although the tetra- 

 merous is also by no means rare ; while 

 the trimerous is generally found in Mono- 

 cotyledons. 



Although a symmetrical flower, as above 

 described, necessarily infers that the parts 

 in each whorl are equal to, or some mul- 

 tiple of one another, still it is very com- 

 mon for botanists to call a flower sym- 

 metrical when the three outer whorls cor- 

 respond in such particulars, while the 

 parts of the gyncBcium are unequal to ^.^ ^^^ Diagram of the 

 them; as m btajj/iy lea 2Ji7inata {fig. 769), novrerotstaphyieapin- 

 where the three outer whorls are penta- ""''^• 

 merous, while the pistil is dimerous. The gyncecium of all the 

 organs of the flower is that which less frequently corresponds 

 in the nimiber of its parts to the other whorls. 



