ANGIOSPERMS. 
613 
The 'zygomorphic and monosymmetrical form occurs, on the contrary, very com- 
monly in those flowers the parts of which are arranged in whorls. A very distinctly 
zygomorphic arrangement is not imfrequently united with a partial or entire abortion of 
particular members, as, e.g., in Columjiea, Fig. 416, and other genera of Gesneracea^, 
where the posterior stamen is transformed into a small nectary; while in Labiatae 
it is entirely wanting* This abortion is 
carried still further in Orchideae, where, 
of the six typical stamens, only the 
median anterior one of the outer whorl 
or the two lateral anterior ones of the 
inner whorl are developed (see Fig. 
410). The ultimate monosymmetrical 
arrangement is sometimes to a certain 
extent indicated by the order of their 
formation, even in the rudimentary con- 
dition of the parts of the flower, when 
their origin is not simultaneous in the 
whorl, and does not progress with a 
definite angle of divergence, but is so 
arranged that the development com- 
mences with one anterior or one pos- 
terior member, and then advances 
simultaneously right and left from the 
median line towards the opposite side 
of the whorl. Examples have already 
been given of this arrangement in 
Papilionacese in the one case and Re- 
sedaceae in the other. 
In the zygomorphic flowers of Fu- 
mariaceae, the diagram (Fig. 411) is, as 
we have already pointed out, symme- 
trically divisible in different ways by 
two planes. The anterior and posterior 
halves, symmetrically similar to one 
another, are unlike the right and left 
halves, which again are symmetrically 
alike. This is the arrangement of the 
parts in the mature flower of Dicentra • 
in Fumaria and Corydalis the right side 
is developed differently from the left, 
one producing a spur^ the other not; 
while the anterior and posterior sides 
remain symmetrical. In this case there- 
fore the plane of symmetry coincides 
with a lateral section. In the zygomor- 
phic flowers of some Solanaceae the plane of symmetry and the median plane intersect 
at an acute angle. But by far the greater number of zygomorphic monosymmetrical 
flowers are so constructed that the median plane coincides with a longitudinal section 
which divides the flow^er symmetrically ; as for instance in Labiatae, Papilionaceae, 
Orchidea;, Scitamineae, Lobeliaceae, Compositae, Delphinium, and Aconitum ^. The zygo- 
morphic development is especially prevalent in the lateral flowers of spicate, racemose, 
Fig. 418. — Zygoniol-phic Ho\Ver Of Orchis jnaciilata : .-1 bad 
divided symmetrically through the middle ; B transverse section 
of the bud ; C horizontal section of the ovary ; D entire mature 
flower, one of the lateral perianth-leaves having been removed ; 
.V mother-axis of the flower; ö bract, j outer, / inner perianth- 
leaves — the posterior one / becomes the labellum, a the single 
stamen, staminodes, i^s gynostemium, // poUinium, h its viscid 
disc (retinaculum), spur of the labellum, y the inferior ovary, 
twisted in D (resupinate) (compare the diagram Fig. 410). 
^ In observations of this kind attention must be paid to torsions, such as occur in the ovary of 
Orchideoe, the flower-stalk of Funiariaceae, the Lal)urnum, &c. 
