NECTAR. 451 



plants apparently depend on the two differing somewhat in con- 

 stitution or character. . . . The mere act of crossing two distinct 

 plants which are in some degree inter-related and which have 

 been subjected to nearly the same conditions does little good 

 as compared with that from a cross between plants belonging 

 to different stocks or families and which have been subjected to 

 somewhat different conditions." 1 



1157. In Volume I. the different methods by which cross- 

 fertilization is effected were sufficiently described, but certain 

 special questions were then purposely left unanswered ; namely, 

 those in regard to the anatomical and chemical nature and the 

 distribution of the attractions by which insects are allured to 

 flowers to insure cross-pollination. 



1158. The nectar which certain flowers offer to insects is made 

 known by color or odor, or both. It is the sweetish liquid com- 

 monly called the "honey" of the flower, secreted by certain 

 specialized organs known as nectar-glands. Mention has already 

 been made (453) of the occurrence of these glands on leaves. 

 In the flower they consist usually of specialized parenchyma not 

 unlike the secreting surface of the stigma (see 1109). They 

 are sometimes raised by a stalk, or adenophore, more or less 

 above the surface of the floral organ on which they are de- 

 veloped, but often not elevated at all. 



1159. Nectar-glands may occur upon any part of the flower, 

 upon its bracts, or even upon some part of the flower-stalk near 

 it. The "Cow-pea" of the Southern States affords a good 

 example of nectar-glands on the flower-stalk. Many species of 

 Euphorbia have them on bracts ; the common Passion-flower 

 and the cotton plant of the South also have them on the same 

 organs. The most remarkable case of arrangement of the glands 

 is found in a tropical plant, Marcgravia nepenthoides ; this has 

 been thus described : " The flowers are disposed in a circle, 

 hanging downwards like an inverted candelabrum. From the 

 centre of the circle of flowers is suspended a number of pitcher- 

 like vessels, which, when the flowers expand in February and 

 March, are filled with a sweetish liquid. This liquid attracts 

 insects, and the insects numerous insectivorous birds. The flow- 

 ers are so disposed, with the stamens hanging downwards, that 

 the birds to get at the pitchers must brush against them, and 

 thus convey the pollen from one plant to another." 2 



1 Darwin : Effects of Cross and Self Fertilization, p. 61. 

 3 Belt ; Naturalist in Nicaragua, 1874, p. 128, 



