488 



BOTANY 



PART II 



may be increased, and tins may aftect the androecium (Hosa), the gynaeceum {Punica 

 granatum), or the perianth {Hagcnin ahyssinica). Another simple modification 

 concerns the position of the whorls. The outer whorl of the androecium, in Erica, 

 for instance, does not alternate with the segments of the corolla but the stamens 

 stand directly above the latter. Such an androecium is distinguished as obdiplo- 

 STEMOxous from the ordinary diplostemonous type. 



Variations in floral symmetry are more important both biologically and with 

 regard to the general habit of the flower. Those flowers which can be divided 

 into similar halves by more than two planes passing through the axis are termed 

 RADIALLY SYMMETRICAL or ACTiNOMORPHic (Fig. 439 A). When a symmetrical 

 division can only be made by two such planes of section the flower is bisymmeteical 

 or bilateral [e.g. Dicentra, belonging to the Fumariaceae). From the originally 

 radial type the dorsiventral (mono-symmetrical, zygomorphic) and the asym- 

 metrical types can be derived by the unequal size of some of tlie members or 



B 



Fig. 439. 



A 



C 



-A, Actinomorphic flower of Geranium sanguineum. B, Zygomorphic flower of Viola 

 tricolor. C, Asymmetrical flower of Carina indica. 



their displacement or suppression (Fig. 439 i?, C ; cf. p. 16. In zygomorphic 

 flowers there is only one plane of .symmetry b}' whicli the flower can be divided 

 into corresponding halves ; while in asymmetrical flowers no such plane of symmetry 

 exists. Dorsiventral flowers tend to take up a particular position in relation to 

 the directive force of gravity and exhibit a high degree of adaptation to insect 

 pollination. According to whether the plane of symmetry coincides with the 

 MEDIAN PLANE of the flower {i.e. the plane passing through the axis of the flower 

 and the main axis), forms an acute angle with the median plane, or is at right 

 angles to it, flowers are distinguished as medianly dorsiventral (Fig. 440 A), 

 obliquely dorsiventral (Fig. 440 £), or transversely dorsiventral respec- 

 tively (Fig. 440 C). Radially symmetrical monstrosities of normally zygomorphic 

 flovi'ers are termed PEi.oRlc. 



If a diagram of tlie arrangement of the members as they are seen in a cross- 

 section of an opened flower-bud is constructed (cf. p. 16) and so oriented tliat the 

 transverse section of the axis of the inflorescence stands above, that of the bract 

 below-, the ground plan of the flower, what is known as a eloral diagram is obtained. 

 The accompanying floral diagram (Fig. 441) is that of a monocotyledouous flower 

 with five whorls of members. A floral formula gives a sliort expression for tliu 



