510 



PART III. THE CLASSIFICATION OF PLANTS. 



Fig. 325.— Floral diagram of 

 Commelyna coelestis showing mo- 

 nosymmetry due to oligomery 

 of the androecium : the plane 

 of symmetry is oblique : the 

 three black stamens are sterile, 

 and the postero-lateral stami- 

 node is often absent : the 

 antero-lateral fertile stamen 

 has a broader connective than 

 the others. (After Eichler.) 



Monosymmetry in consequence of an oligomerous androecium 

 is less common ; a good instance is 

 afforded bj Commelyna coelestis, in which 

 flower the plane of symmetry is oblique 

 (Fig. 325). Monosymmetry is also cha- 

 racteristic of irregular flowers, whether 

 eucyclic or heterocyclic ; of flowers, that 

 is, in which the members of one or 

 more whorls differ in various respects 

 among themselves, accompanied fre- 

 quently by a reduction in the typical 

 number of members in one or other of 

 the whorls, frequently of the androecium: 

 it is, in fact, to irregular flowers that 

 the term zijgomorphic is specially applied 

 in Descriptive Botany. Such a flower 

 usually presents a clear distinction into 

 two diverse portions, an anterior and a 

 posterior, separated by the lateral plane, whilst the two lateral 

 halves about the median plane are symmetrical ; hence it is 

 clearly dorsiventral (Fig. 326). 



Dorsiventrality is presented by some flowers which, so far as 



their early development is 

 concerned, or even so far 

 as is shown by their floral 

 diagram, are actinomorphic, 

 isobilateral, or simply zy- 

 gomorphic, the dorsiven- 

 trality being due to the 

 subsequent irregular de- 

 velopment of some of the 

 floral leaves ; as in some 

 eucyclic flowers (e.g. among 

 Monocotyledons, Agapan- 

 thus, Alstroemeria, Ama- 

 ryllis, Gladiolus ; among 

 Dicotyledons, Dictamnus, 

 and other Rutese, species of 

 Impatiens, Pelargonrum), 

 and in some heterocyclic 

 flowers (e.g. some Scrophulariaceee, Labiatae, some Caprifoliacese, 



Fift. 826.— DoTBi ventral flower of a Heracleum 

 (mag. 



