73 
THE FLOWER (. SYMMETRY) 
Floral Symmetry is another feature that shows much 
variety in its evolution. In a good many flowers the 
number of members in each whorl is the same, and each 
member of any whorl is like every other member of the same 
whorl in size, shape, texture, &c. The symmetry is radial 
(p. 31) or actinomorphic , and the flower is regular , e.g. in 
the Ranunculaceae, Rosaceae (see diagrams in Part II.), 
Crassulaceae, Ericaceae, &c. It more frequently happens 
than not that the symmetry, perfect in the outer whorls, is 
disturbed by the fact of the gynoeceum possessing too few 
carpels, as in most Sympetalae. Such flowers, however, are 
generally termed actinomorphic also. But if one or more 
members be missing from any of the outer whorls, or if in 
any whorl all the members are not exactly alike, then the 
flower is irregular. If it can be divided symmetrically 
in any plane, it is zygomorphic (see floral diagrams of 
Papaveraceae, Labiatae, Scrophulariaceae, &c.) ; if it cannot 
be so divided it is asymmetrical as in Valerianaceae. 
The flower usually stands in the axil of a bract and as a 
rule the same side of each flower on any plant faces the 
bract. The side facing the bract is anterior, that facing 
away from it, i.e. towards the original stem, is posterior . 
The upper side of a floral leaf, as of a vegetative leaf, is 
ventral, the lower dorsal. 
A great many families, e.g. Ranunculaceae, contain both 
regular and irregular flowers, whilst others, e.g. Labiatae, 
contain only the latter. The change from regularity to 
irregularity is then one which has begun independently in 
many branches of the phanerogamic tree (p. 29). Of the 
determining and active causes we know little, but we may 
easily see the advantages of the change. Irregular flowers 
usually stand more or less horizontally, whereas regular ones 
are most often, perhaps, erect. A flower that stands 
horizontally will obviously protect its pollen better, at least 
in those stamens that are uppermost. Further, the only 
convenient track for the entering insect is by the lower 
side. We can easily imagine the evolution of such a flower 
into a zygomorphic one. The lower stamens and the style 
might bend over to come under the upper side of the flower, 
and this might develope into a protective hood, whilst the 
lower half of the flower (usually only the corolla) might 
