THE ANGIOSPERAIAE 1113 



total number of petals and stamens in the normal flower. The double 

 Buttercup is a good example and Pliny's " hundred-petalled Rose " {R. 

 centifolia) is another. To an older generation of morphologists the phe- 

 nomenon was known as "petalomania". 



A minor anomaly, which may be included under the heading of doubling, 

 is that sometimes seen in Primula and other sympetalous flowers, where an 

 abnormal, petaloid calyx forms an exterior " corolla ", enclosing the normal 

 one and producing what gardeners call the " hose-in-hose " varieties. 



The physiological causes of doubling are still obscure, but high nutrition 

 seems to favour it, and although double flowers are sometimes found wild, 

 their low fertility is a hindrance to their survival in that state. Doubling 

 is commoner in cultivation, where, on the contrary, the artificial taste of 

 florists ensures their perpetuation. The tendency is hereditary in certain 

 strains, and if a double flower yields good seed, its progeny frequently 

 produce a high percentage of doubles, e.g., about 60 per cent, in the garden 

 Stock (Matthiola). The character is not, however, genetically fixed and its 

 expression depends on maintaining good cultivation and rapid growth, 

 without which the majority of the seedlings will only display the single 

 form. 



Some species of plants habitually produce flowers of more than one 

 form on the same individual. This is called heteranthy. Many instances 

 occur of variation in the distribution or development of the sporogenous 

 organs among flowers on the same plant (see p. 1269), but such variations 

 do not as a rule affect the general model of the flow-ers, though male and 

 female flowers in monoecious and dioecious species may sometimes differ 

 considerably in aspect, as, for example, in Akebia (Fig. 1153), a well-known 

 climber, the female flowers of which are twice as large as the male flowers. A 

 further example is provided by Albizzia moJuccana (Leguminosae) which 

 has inflorescences with large, terminal, female flowers and small, lateral, 

 male flowers. 



A series of genera in the family Malpighiaceae have an exceptional kind 

 of heteranthy, for the plants bear large numbers of reduced flowers as w^ell 

 as the normal ones. The former have no calyx, a small corolla or none, one 

 abortive stamen and two carpels with an abortive style. There is here no 

 question of cleistogamy, for these flowers are wholly sterile. 



Variations of the flower model,* i.e., true heteranthy, are usually asso- 

 ciated with different positions in complex inflorescences. For instance, it is 

 well known that in many Compositae there is a wide difference between the 

 flowers which form the central " disc ", e.g., in the Sunflower {Helianthus), 

 and the marginal flowers. The former are actinomorphic and bisexual, 

 but those on the margin, called the " ray florets ", are either sterile or only 

 female and have a relatively enormous lateral expansion of the corolla, 

 or part of it, towards the exterior, the effect being to surround the circular 



* We have used the word " model " in an effort to give an Enghsh equi\alent for the 

 German " Gestalt ". The latter has secondan.' implications which are not entirely covered 

 by the English word but we believe it conveys the main meaning of a somewhat difficult term. 



