1336 A TEXTBOOK OF THEORETICAL BOTANY 



stigma. When the flower first opens the stigma occupies the mouth of the 

 corolla tube, while the anthers lie further down and are not yet mature. At 

 this stage a moth will therefore pollinate the stigma if it has previously 

 visited another flower. Later the anthers mature and pollen will be deposited 

 on the tongue of an insect as it is thrust down the corolla tube. Pollen so 

 collected will be deposited on the stigma of a younger flower as the moth 

 seeks for the narrow opening to the nectar which collects in the base of the 

 corolla tube. 



A unique case of moth pollination is afforded by the Yucca and its 

 pollinator the moth Pronuha yiiccasella, which we have already described on 

 p. 1263. So far as is known no other insect can effect pollination and indeed 

 in those parts of the world where Pronuba does not occur, Yucca planted 

 in gardens never sets seed. 



iii. Dipterous Flowers 



The Diptera are only feeble fliers and often effect pollination by crawl- 

 ing into flowers. Moreover in many cases it is doubtful if they obtain any 

 nourishment from the flower and their reasons for visiting them may vary. 

 Some are attracted by the carrion smell, which suggests dead meat, which 

 they favour as a place to lay their eggs. Others visit the flowers for warmth 

 or for protection from wind or rain. Others again are caught by some 

 deceptive mechanism provided by the flower so that the animal can scarcely 

 be said to be a willing visitor at all. Indeed, so diverse are the circumstances 

 of fly pollination that it is more difiicult to circumscribe this group than any 

 of the previous ones. Six distinct types of mechanisms are recognized. 



{a) Nauseous Flowers 



The flowers which belong to this group usually have their nectar either 

 completely exposed or only partly concealed and in this respect they show 

 little specialization. In colour they are dull brown, purple or yellow, often 

 spotted and generally suggestive of decaying meat, a feature which is 

 emphasized by the nauseous odour which they emit. Such flowers are 

 visited by carrion and dung flies, but they also attract many other insects. 

 Further it must be remembered that flowers which emit odours not specially 

 unpleasant to man may attract flies in the same way as the more nauseous 

 odours do. It is therefore by no means easy to limit sharply the types 

 which should be included in this section. For example, the flowers of 

 Umbelliferae are generally fly-pollinated, for which their open form and the 

 shallow layer of nectar they secrete are well suited. The flowers of many 

 species have heavy aminoid odours attractive to flies, which sometimes 

 frequent the plants in swarms, e.g., Smyrnium olusatrum, but not all these 

 odours could be classed as nauseous, nor is there usually the murky colour- 

 ing which goes with truly nauseous odours. 



Among the more common and simple types which belong to this class 

 are various species of the genus Saxifraga. Miiller, studying the alpine 

 flora, found many species of this genus were pollinated by flies and we may 



