THE ANGIOSPERMAE 1097 



diaceae. This theon,' is supported by observations on the flower primordia 

 and by the comparison of related genera. In Stellaria media variations of the 

 normal flower occur, with petals and antipetalous stamens suppressed. This 

 is paralleled by the normal structure in other genera. In Corrigiola littoralis 

 the antipetalous stamen whorl is absent; in Scleranthiis perennis the petals 

 are absent and sometimes also the antipetalous stamens; in Paronychia 

 species both sets of organs are absent. 



The alternation rule naturally does not apply to spiral flowers, nor does 

 it strictly apply where the number of parts in successive whorls is not the 

 same. The carpels, for example, are often fewer in number than the stamens, 

 and no fixed rule applies to their relative positions. There is usually the 

 nearest approach to alternation that is compatible with the svmmetry of the 

 particular flower, for which the mechanical necessities of space-filling in the 

 limited room available may be held accountable. 



All flowers in which the arrangement of parts around the axis is sym- 

 metrical about at least two planes are called actinomorphic. That this is 

 the fundamental type of flower symmetry is witnessed by two facts; in the 

 first place by the rule of equidistance, which is no more than the observed 

 fact that the insertions of floral parts are normally equally spaced around the 

 receptacle, and by the further observation that even in irregular flowers 

 the receptacle itself is normally circular in section, exceptions, such as the 

 oval receptacle of Reseda, being relatively rare. Thus the earliest stages 

 of development are normally actinomorphic, even in flowers where this is 

 afterwards changed. 



Actinomorphic symmetry is maintained even when the numbers of 

 parts in the floral whorls are variable. This may be seen by comparing 

 terminal and lateral flowers in some inflorescences {e.g., Berheris and 

 Sanguisorha) where the terminal flowers often have an increased number of 

 parts. Variations may also arise from chorisis (see p. 1105) and in other 

 ways. The situation is met by the redistribution of parts around the recep- 

 tacle so that equidistance is maintained and the circle of 360° is equally 

 divided between the parts. 



Radial symmetry is, however, replaced in many species by bilateral 

 symmetry about a single median plane, which may be antero-posterior, 

 transverse, or oblique, such flowers being called zygomorphic. Some rare 

 cases occur of asymmetric flowers, chiefly among the Marantaceae, in 

 which there is no plane of symmetr}'. 



The irregularities which produce zygomorphy are of four distinct kinds, 

 which may be present separately or, more often, together. The following 

 are instances in which they occur separately, (i) Reduction in the number 

 of one category of parts in relation to the others. Some of these cases are 

 doubtful. For example, the partial sterilization of three stamens out of 

 six in Cotnmelina coelestis or the suppression of two carpels out of three in 

 Viburnum lantana (Fig. 1066) creates zygomorphy in these flowers, but 

 the rest of the parts retain radial symmetry. The zygomorphy in such cases 

 is not obvious to the eye and is only to be discovered by careful analysis. 



