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 Columnea, Fig. 416, and other genera of Gesneraceae, 

 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 

 Papilionaceae 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 flower symmetrically ; as for instance in Labiatae, Papilionaceae, 

 Orchideae, Scitamineae, Lobeliaceae, Compositae, Delphinium, and Aconitum'^. The zygo- 

 morphic development is especially prevalent in the lateral flowers of spicate, racemose, 



Fig. 418. — Zygomorphic flowfer of Orchis in&cuMta : A bud 

 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 ; 

 X mother-axis of the flower ; b bract, s outer, / inner perianth- 

 leaves — the posterior one I becomes the labellum, a the single 

 stamen, st staminodes, gs gynostemium, // poUinium, h its viscid 

 disc (retinaculum), sp 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 

 Orchidece, the flower-stalk of Fumariaceae, the Laburnum, &c. 



