512 PART III. — THE CLASSIFICATION OF PLANTS. 



symmetry coincides with the median plane the symbol -^ is used, 

 and when it coincides with the lateral plane the symbol ->. 



Sometimes regular flowers are developed by plants which 

 usually produce irregular flowers : these exceptional flowers are 

 termed peloria. This is due in some cases to the fact that the 

 primitive number and arrangement of the floral organs is not 

 disturbed by the irregular development of the parts which usually 

 takes place : such cases are distinguished as regular peloria (e.g. 

 Viola, Gloxinia, Labiatae, etc.) In other cases the peloric flower 

 is to some extent the result of the symmetrical development of the 

 irregularity (e.g. the development of five spurred petals and five 

 stamens in Linaria). Dorsiventral flowers are, generally speaking, 

 such as are borne laterally on the inflorescence ; whilst the ter- 

 minal flowers (which may be regarded as peloric) are frequently 

 regular. Peloric lateral flowers are, however, known to occur. 



There remain to be considered those flowers which cannot be 

 symmetrically divided in any plane : such flowers are asymmetric. 

 Amongst these are to be included most of the acyclic or hemi- 

 cyclic flowers in which the number of members is high and the 

 divergence variable (e.g. Calycanthus, some Ranunculaceae, etc.) : 

 the asymmetry of most of these is approximately, though not 

 quite accurately, actinomorphic, but in some it is dorsiventral (e.g. 

 Delphinium, Aconitum). Asymmetry is rare in cyclic flowers, 

 but is to be found in some heterocyclic flowers : for instance, 

 in Tropseolum, Canna and other Marantaceae, Valeriana and 

 other Valerianaceae, where the asymmetry is dorsiventral and is 

 due to oligomery and irregularity combined, whilst in other cases 

 (e.g. some Paronychieae, Fig. 328 C) it is due merely to oligomery. 



The Floral Organs. 



The Perianth is completely absent, that is, the flower is achlamy- 

 deousj in a few families (e.g. Piperaceae, Araceae, Graminaceae, many 

 Cyperaceae, Salicaceae). When present, it is usually differentiated 

 into calyx and corolla, the flower being termed dichlamydeous or 

 hiseriate: when the calyx and corolla clearly differ from each other 

 in colour, texture, etc., the flower is said to be heterochlamydeous ; 

 for instance, when the calyx is green and the corolla highly 

 coloured (as in most Dicotyledons, and in some Monocotyledons 

 such as Tradescautia and Commelyna) ; or when the calyx is 

 coloured (petaloid) and the petals reduced to nectaries (as in Helle- 

 borus and other Ranunculacese). When the perianth-leaves are all 

 alike, the flower is said to be homochlamydeous. This condition 



