1338 A TEXTBOOK OF THEORETICAL BOTANY 



able case is that of the flowers of the genus Rafflesja, especially R. arnoldi, 

 which has a flower nearly a yard across and emits a most nauseous odour. It 

 is said to be pollinated by carrion flies. An account of its features will be 

 given under the family Rafflesiaceae (see p. 1707). 



Among the other flowers which belong to this class are Smilax herbacea, 

 Trillium erectum and Calla paliistris. A peculiar case is that of Cohaea 

 scandens, a rapidly growing climber often cultivated in gardens. It is a 

 member of the Polemoniaceae. The large bell-shaped flowers at first emit a 

 nauseous odour and are visited by carrion flies. Later this odour disappears 

 and is replaced by sweet-smelling nectar. In this condition the flowers are 

 sought after by butterflies. 



{h) Pitfall Flowers 



This term was used by Knuth to include various types of pollination 

 mechanisms by which small flies are imprisoned. For one reason or another 

 they find their way through a small opening in the floral organs and once 

 inside they are trapped. Their prison may consist of a single flower or it 

 may be an inflorescence and composed of large numbers of separate 

 flowers. In either case the function of the fly is to be covered with pollen, 

 which it brushes off on the stigma of the same or of another flower. Gener- 

 ally some part of the flower is chocolate brown in colour, a colour to which 

 these small flies are attracted. What induces the insect to enter the flower 

 is not always clear. Frequently it may be to obtain protection, for the tem- 

 perature inside such structures as the spathe of Arum maculatum may be 

 several degrees higher than that outside, so that warmth may at times be 

 what these insects seek. Only very rarely is nectar secreted and few of these 

 pitfall flowers reward the insects which visit them. 



Pitfall flowers have probably been derived from nauseous ones and we 

 may describe Asarum europaeum (Aristolochiaceae) as an intermediate type 

 between the two (Fig. 1248). The plant is widely distributed and occurs 

 occasionally in Britain, although not native. The flowers are inconspicuous 

 and are borne in terminal inflorescences on creeping leafy shoots. Each 

 flower is brownish on the outside and a dark, dirty purple within. It emits 

 a strong odour likened to camphor. The ovary is inferior and above it rises 

 a three-partite perianth with the lower halves of its segments joined to form 

 an open cup. The perianth is lined with downward-pointing hairs. A short 

 united style branches at the top into six arched segments, around and 

 between which arise twelve stamens, whose long, pointed apices bend 

 inwards to meet over the centre of the style. Small flies work their way down 

 between the stamens and the arms of the style and find themselves there in 

 a cage, escape laterally from which is prevented by the perianth hairs. There 

 they stay until the anthers have shed the pollen, when the stamens bend 

 outwards and the pollen-dusted flies are freed to go on to another flower. 

 The flowers are protogynous, thus cross-pollination can only be ensured by 

 flies which come from an older flower to a younger one. 



This relatively simple example leads to the more elaborate mechanisms 



