1 1 14 A TEXTBOOK OF THEORETICAL BOTANY 



inflorescence with a ring of petaloid " rays ", which simulate single petals 

 in general appearance and even in details of structure (see Fig. 1878, p. 1948). 

 The whole inflorescence thus acquires the aspect of a single flower and may 

 be biologically regarded as conforming to the flower " model ", though 

 morphologicallv quite distinct. 



Sterile marginal flowers are characteristic of the genus Viburnum 

 (Fig. 1089) (especially in V. opulus, the Guelder Rose), which has corymbose 

 inflorescences. In this case the expansion of the corolla is sometimes 



Fig. 1089. — Vibiiniinii tomentosum. Marginal flowers with 

 irregularly expanded perianths. 



unsymmetrical but in some species it is symmetrical and the sterile flowers 

 retain their radial symmetry, though many times larger than the fertile, 

 central flowers. An analogous appearance is presented by the inflores- 

 cence of Hydrangea, though in this case it is a petaloid calyx which becomes 

 expanded. The favourite tropical shrub Mussaenda owes its attraction 

 to the enormous development of one coloured sepal from each of two 

 or three flowers on the margin of the corymbose inflorescence. 



Excessive development of the corolla in marginal flowers, though it 

 seems to depress the development of other parts, is not however invariably 

 associated with sterility, for the zygomorphic marginal flowers in Umbel- 

 liferae referred to on p. 1156 are generally fully fertile (Fig. 1090). 



Excessive or specialized development on the periphery of an organ or 

 group of organs like an inflorescence, is sometimes called exotrophy. It is 

 not confined to such cases of perianth development as we have mentioned, 

 but may be also observed in bracts and fruits and in vegetative organs such 

 as asymmetrical leaves. 



Positional heteranthy is likewise not limited to cases of exotrophy. For 

 example, in Muscari comosum, the terminal group of flowers in the raceme 

 is sterile, each flower being elongated and of a brighter blue than the fertile 



