PHYSIOLOGY OF THE FLOWER. 121 



instance, that nectar, whenever present, is powerfully odorous 

 from the insect point of view, even when in minute quantities. 

 A jar of honey possesses a well-marked odour, easily perceived 

 by us, but a minute drop of the same appears scentless ; yet to 

 an insect such a drop is relatively large. Nectar that is com- 

 pletely hidden from sight will thus be smelt out, just as wasps 

 rapidly find their way through the open window of a room con- 

 taining sweet stuffs. 



A powerful scent is often more attractive to insects than bright 

 colour or large size, as may be well seen by comparing certain 

 closely allied forms. Thus, the sweet violet is visited much more 

 frequently than the larger and brighter, but scentless, pansy. 

 Similarly, the small field bindweed {Convolvulus arvensis) is a 

 much greater favourite with insects, owing no doubt to its 

 fragrance, than the large white odourless form, Calystegia sepium. 



The willow and lime (cf. p. 1 1 6) are good examples of inconspicu- 

 ous but insect-pollinated flowers, which attract by fragrance. 



Mignonette also makes up for its insignificant appearance by 

 a fragrant smell and the excretion of abundant nectar on the 

 back of a sort of plate which projects from the floral receptacle. 

 It is a favourite with certain bees. 



Flowers pollinated by night-flying insects reserve their fra- 

 grance for this time. 



The preceding features concern the attraction of insects ; the 

 following have to do with their reception. 



Irregularity. — This always has reference to insect visitors. 

 By it a landing-stage is generally provided, as in labiate flowers, 

 and it may also have to do with the excretion of nectar, as in the 

 spur of toad-flax, or the storing of nectar, as in pansy (fig. 51). 

 The landing-stage is always in such a position that the insect is 

 brought into contact with stamens and stigma. If a part of its 

 body gets dusted in one flower, things are so arranged that this 

 part will touch the stigma of some other flower, and so on. It 

 is also to be remembered that irregular flowers lay themselves 

 out to secure the services of special insects, and, in such cases, 

 correlated modifications of structure are found in insect and 

 flower. This may be advantageous, as insects are often kept in 

 this way to the same kind of flower, and therefore effect cross- 

 pollination more certainly than they otherwise would ; but there 

 is a corresponding disadvantage. Thus, red clover sets no seed 

 in some of our colonies, owing to the absence of humble-bees, by 

 which, in this country, it is usually pollinated. This flower also 

 shows in what a complex way organisms are linked together, for, 

 to use a well-known illustration by Darwin, red clover is depen- 

 dent on cats for the formation of its seeds. These animals destroy 



