206 THE SCENT OF FLOWERS AS A MEANS OF ATTRACTING ANIMALS. 



certain materials. Thus vibrations which exceed the limits of irritability of the 

 olfactory nerves in rapidity produce no smell. 



Whichever hypothesis one accepts one comes to the conclusion that a great 

 difference may exist between the sense of smell of men and animals according to 

 the different degree of sensitiveness of their olfactory fibres. Although the 

 molecules of a substance flioating in the air stimulate (i.e. set in motion) no single 

 nerve-ending in the human olfactory mucous membrane, this does not prove the 

 absence of nerves in the olfactory organ of some animal sensitive to the particular 

 form of motion of these molecules. It might easily happen that one insect would 

 smell Hyacinths but not Roses, while another would smell Roses and not Hyacinths. 

 This conclusion is, however, of importance in explaining the allurement of certain 

 animals to flowers which appear scentless to man, as well as in explaining the 

 phenomenon that many flowers are eagerly visited by one group of insects and 

 are avoided or rather ignored by another. The Virginian Creeper, Ampelopsis 

 quinquefolia, so often planted to cover porches, palings, and walls, develops 

 flowers in midsummer which are visited by bees very industriously and eagerly. 

 The colour does not act as an allurement in this case, for the flowers have green 

 corollas, are hidden away under the foliage, and cannot be seen even by good eyes 

 at a little distance. Yet the bees fly thither from all sides in such a way as to 

 leave no doubt that the flowers of the Am'pelopsis can be perceived by them a 

 considerable way off. Since it is not their appearance it must be their smell which 

 announces their presence! But to men they appear to be quite scentless! The 

 flowers of the Common Bryony {Bryonia dioica) are not less remarkable. They 

 occur on two kinds of plants, i.e. on one plant are developed only staminate and on 

 the other only pistillate flowers, and since the pollen is not powdery, and therefore 

 not scattered by wind, it must be carried by insects from plant to plant if the 

 ovules are to mature. But the flowers, especially the pistillate ones, are very 

 insignificant, green in colour, with faint smell, and they are half hidden under the 

 foliage. Many insects fly past them without noticing them. They are almost 

 exclusively visited by one of the Hymenoptera, viz, And7'ena Jiorea, and it can find 

 them even in the most out-of-the-way places. This can hardly be accounted for 

 except by supposing that the scent of Bryony flowers is perceived by these 

 particular bees and not by other insects. To these two examples of insignificant 

 flowers, which appear to men and to many animals to be scentless but which are 

 nevertheless eagerly tracked by certain insects, may be added the common Birth- 

 wort (Aristolochia Clematitis), the Whortleberry (Vacciniuin Myrtillus), ChamcB- 

 orchis alpina, the Twayblade (Listera ovata), and many others. It is probable 

 that there are also flowers which differ from these in having bright colours 

 contrasting with the green foliage, and in addition exhale a special scent to allure 

 certain animals. It is, of course, hardly possible to speak with certainty. In all 

 these questions w^e have to deal with observations concerning the relations between 

 insects and flowers in nature, and since many sources of error exist, the conclusions 

 arrived at must be accepted with discretion. As to the so-called " flower fidelity " 



