1 156 A TEXTBOOK OF THEORETICAL BOTANY 



usually directed more or less vertically, whereas in typically zygomorphic 

 flowers the opening ofthe flower is usually directed outwards and downwards. 

 Such flowers are characteristic of the inflorescences of Umbelliferae and 

 Compositae. Their function is apparently the increase of the conspicuous- 

 ness of the whole inflorescence, which, in the cases cited, as in others, 

 consists of small, individually inconspicuous flowers. (See Fig. 1090.) 



Another form of functional zygomorphy is due to the development of 

 nectar spurs, in the form of hollow sacs, as projections from one or more 

 of the perianth parts. In rare instances, as in some Crucifers, spurs are 

 developed symmetrically and there is no zygomorphy, but it is usual to find 

 one spur only, developed by the invagination of one sepal or petal. The 

 abnormality called peloria (see p. iioo), namely the secondary develop- 

 ment of radial symmetry in a zygomorphic flower-type, is sometimes 

 brought about by the production of extra spurs from all the members of a 

 perianth whorl, instead of from one only. This is exhibited, for example, by 

 some strains of Linaria nilgaris, in which the abnormality is inherited. 

 The additional spurs compensate the natural zygomorphy and produce an 

 actinomorphic flower. 



Peloric flowers are usually produced on orthotropic axes, particularly 

 the main axis, in species with otherwise zygomorphic flowers. Although 

 frequently exhibited by spurred flowers it is not confined to them and a 

 similar regular development of the petals in zygomorphic corollas is some- 

 times seen in Viola and quite frequently in certain strains of Digitalis (see 

 Fig. 1070). The opposite case, namely the development of zygomorphy 

 in species whose flowers are otherwise actinomorphic, is rare, but examples 

 have been recorded, every case being in flowers growing on horizontal 

 branches. A notable instance of such zygomorphy has been reported from 

 Fuchsia coccinea. 



A spur may be no more than a slight concavity or it may be a tubular 

 extension several inches long. The longest are found in the orchid Angrae- 

 cum sesqiiipedale, a native of Madagascar, whose spurs are more than a foot 

 long. This limits access to its nectar to the longest-tongued Lepidoptera, 

 with which therefore the pollination of the plant is linked so closely as 

 almost to amount to a symbiosis. 



Sepal spurs are less common than petal spurs. The Cruciferae, men- 

 tioned above, show them almost throughout the family. Several members 

 of the Ranunculaceae also have them, e.g., Myosiinis, in which each sepal 

 is spurred, and Delphinium, in which the posterior sepal and petal are both 

 spurred, the one fitting inside the other. The garden Tropaeolum has 

 a long, coloured spur which is composed of prolongations from the three 

 posterior sepals fused together. 



Petal spurs are too numerous to be specified, but we may note that they 

 occur both in polypetalous corollas, e.g., Viola, Aquilegia (Fig. 1129), and 

 m sympetalous corollas, e.g., Centranthus and Linaria. 



That zygomorphy depends on the differential development of certain 

 sectors ot the flower axis is clearly indicated by these spurred flowers, thus 



