CHAP. X. MEANS OF CKOSS-FERTILISATION. 373 



Not only do the bright colours of flowers serve to 

 attract insects, but dark-coloured streaks and marks 

 are often present, which Sprengel long ago maintained 

 served as guides to the nectary. These marks follow 

 the veins in the petals, or lie between them. They 

 may occur on only one, or on all excepting one or 

 more of the upper or lower petals ; or they may form 

 a dark ring round the tubular part of the corolla, or 

 be confined to the lips of an irregular flower. In the 

 white varieties of many flowers, such as of Digitalis 

 purpurea, Antirrhinum majus, several species of Dian- 

 thus, Phlox, Myosotis, Rhododendron, Pelargonium, 

 Primula, and Petunia, the marks generally persist, 

 whilst the rest of the corolla has become of a pure 

 white ; but this may be due merely to their colour 

 being more intense and thus less readily obliterated. 

 Sprengel's notion of the use of these marks as guides 

 appeared to me for a long time fanciful ; for insects, 

 without such aid, readily discover the nectary and 

 bite holes through it from the outside. They also 

 discover the minute nectar-secreting glands on the 

 stipules and leaves of certain plants. Moreover, some 

 few plants, such as certain poppies, which are not 

 nectariferous, have guiding marks ; but we might 

 perhaps expect that some few plants would retain 

 traces of a former nectariferous condition. On the other 

 hand, these marks are much more common on asymme- 

 trical flowers, the entrance into which would be apt to 

 puzzle insects, than on regular flowers. Sir J. Lubbock 

 has also proved that bees readily distinguish colours, 

 and that they lose much time if the position of honey 

 M hich they have once visited be in the least changed.* 

 The following case affords, I think, the best evidence 



Briti.-ili WM F'r wors 'n relation to Insects,' 1875, p. 14. 



