i3o8 A TEXTBOOK OF THEORETICAL BOTANY 



sensations in an insect from those they cause in ourselves. Just as a dog can 

 recognize and distinguish smells which are imperceptible to man, so may 

 odours which we cannot appreciate be detectable by insects. The flowers 

 of Ampehpsis are hidden from sight by the leaves and, unlike its relation the 

 Vine, they have no perfume detectable by man, but they are regularly 

 thronged with insects, including Hive Bees, which must have been guided 

 to them by their perception of an odour. (See also p. 1266.) 



It has been noted that some flowers are scentless by day, but emit a 

 strong odour at night. In almost every recorded instance such flowers are 

 pollinated by night-flying moths. It is therefore reasonable to assume that 

 moths are able to recognize these odours and may be guided to the flowers 

 therebv. 



Secretion of Nectar 



The nature and position of nectaries both as floral structures (see p. 1244) 

 and also extra-floral glands (see Volume I, p. 472) have already been dis- 

 cussed elsewhere. The question of nectar secretion and the factors influen- 

 cing its discharge have also been dealt with (see p. 1257), so that we need 

 only now refer to this very important factor in pollination. 



Edible Sap 



Certain flowers, for example Orchis morio and O. mascula, have the appear- 

 ance of nectar-secreting flowers, but on examination are found to possess no 

 nectaries. In these instances it has been shown that there is in the spur a 

 tissue which contains a juice attractive to certain insects. In these examples, 

 as in other flowers, such as Hyacinthus orientalis and Erythraea centaiirium, 

 where the petals provide the juice, bees and butterflies bore into the tissues 

 and suck the sap. In certain species of Portulaca tiny knobs are produced 

 on the floral disc which become distended with sap, while in species of 

 Verbascum, Anagallis and Tradescantia succulent hairs are developed on the 

 stamens, which are eaten by insects, or they may secrete a sticky substance 

 which becomes adherent to the visitors and causes pollen to stick to them. 

 Such flowers, which provide a juice, other than nectar, which attracts 

 insects, are referred to as False Nectar Flowers. 



Shape of the Flower 



The shape of the corolla has an important bearing upon the method of 

 pollination. In flowers with tubular corollas there is a direct correlation 

 between the length of the tube and the length of the proboscis of the visiting 

 insect. There are many tubular flowers whose nectar can only be reached | 



by long-tongued insects. Flowers of this type do not invite any other kind of 

 visitor and thereby they avoid the robbing of their nectar by insects which 

 by their shape or size cannot perform pollination. In some flowers, in which 

 the corolla is completely closed, entry is only possible to insects sufficiently 

 large or strong to be able to force apart the petals, as in Antirrhinum, where 

 entry is precluded to all insects other than Humble Bees. 



Many zygomorphic flowers provide a platform upon which the insect 



