NECTAR OF FLOWERS 



97 



such flowers they disappear more or less completely in the double variety, where they 

 have become mere useless markings. Insects are certainly attracted from a distance 

 by these flowers, but as soon as they come near they notice that here there is nothing 

 for them (for as a rule the formation of pollen and the secretion of nectar cease when 

 flowers become double), and turn away without searching for food. 



Fig. 14. Nectar-guides, (i) Dotted marks of Saxifraga aspera L. (2) Ring-shaped nectar-guide 

 of Myosotis alpestris Schmidt. (3) Spots and streaks of Gentiana acaulis L. (4) Streaks on tiie lower 

 lip of Teucrium Chamaedrys L. (5) Streaks and ring-shaped nectar-guide of Veronica Chamaedrys. 



Delpino ('Ult. Oss., Atti Soc. ital. sc. nat.,' Milano, xvi, 1874, pp. 234 et seq.) 

 distinguishes between intra-floral nectaries, occurring in the flower, and circum- 

 floral nectaries on its outer side, as e.g., in Euphorbia, where the honey is secreted 

 by crescentic or rounded appendages of the cup-shaped involucre. There may also 

 be extra-floral nectaries outside the flower altogether, but situated near it, as in the 

 case of the above-mentioned (pp. 73-4) Marcgraviaceae, which investigations of 

 Thomas Belt ('The Naturalist in Nicaragua,' 1874) made known to us. In this 

 family nectar is secreted by bracts of striking form and colour by which the 

 humming-birds that effect pollination are attracted. 



Besides these three kinds of nectaries occurring in the region of the flower, and 



