B04 SYMMETRY OF THE FLOWER. 
A symmetrical flower in which the number of parts in each 
whorl is the same, as in Crassula (fig. 785), is said to be isomerous, 
or when the number is unequal, as in the Rue (figs. 579 and 611) 
and Sedum (fig. 786), the flower is anisomerous. The number 
of parts is indicated by a Greek numeral prefixed to the word 
meros, signifying a part. Thus, when there are two parts in 
the whorls, as in Circea ( fig. 787), the flower is dimerous, and 
the symmetry is said to be binary or two-membered. This may 
be considered either as answering to the distichous or two-ranked 
arrangement of leaves (see page 151); each whorl forming a 
cycle composed of two organs, the internodes between them not 
being developed ; or to successive pairs of opposite leaves de- 
cussating with each other. This arrangement is thus marked, 
s/. When there are three parts in a whorl, as in the Squill 
(fig. 28), 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 (page 152), each whorl form- 
ing a cycle of three organs, the internodes between them not 
Fic. 787. 
Fig.787. Diagram of the flower of Cirecea ——Fig.788. Diagram of the flower 
of Staphylea pinnata, 
being developed ; or to successive whorls of three organs in 
each. When there are four parts in a whorl, as frequently in 
the Rue (fig. 579), the flower is tetramerous, and the symmetry, 
which is marked 4/, is quaternary or tetragonal ; the successive 
whorls in such a flower may be compared directly with whorls 
of leaves each consisting of four organs; or indirectly with 
opposite decussating leaves combined in pairs, the internodes 
not being developed. When there are five parts ina whorl, as in 
Crassula rubens (fig. 785), the flower is said to be pentamerous, 
and the symmetry, which is marked thus, 8/, quinary or penta- 
gonal. Such a flower may be considered as answering to the 
pentastichous arrangement of leaves (page 150) with the inter- 
nodes undeveloped ; or to be composed of successive whorls of 
five leaves, the internodes between each whorl being almost un- 
developed, or very short. 
Of the above arrangements, the pentamerous is most common 
among Dicotyledons, althouzh the tetramerous is also by no 
