134 



MORPHOLOGY 



dacece a.nd Orchidacece plants have pinch-trap flowers. 

 For example, Asclepias curassavica (fig. 1 18), an erect 

 herb of waste places, with handsome orange-red 

 flowers, is a typical example of this condition. 



Of all pollinating insects the bee and the butterfly 

 are more highly specialized than the rest, and the 

 flowers for which they have 

 special preference are likewise 

 highly specialized. Moreover, 

 these insects show distinct colour 

 preferences. For instance, glar- 

 ing colours, especially bright- 

 yellow, are least agreeable to the 

 honey-bee, while saturated blue 

 is most attractive to it. A series 

 has been constructed in which 

 colours which are appreciated 

 by bees have been placed in the 

 orderof their preference, namely, 

 saturated blue, violet, blue, red, white, and pale- 

 yellow, pure green, glaring red, and yellow. Simi- 

 larly, butterflies markedly prefer dark colours to 

 bright ones. It has already been mentioned that 

 certain butterflies prefer flowers resembling their own 

 wings in colour. 



Fig. ii8. — Asclepias curassa- 

 vica, after removal of calyx and 

 corolla 



^0, two pairs of pollinia catching 

 the leg by clips (after Knuth) 



CHAPTER XVIII 



STRUCTURE OF POLLEN-GRAINS AND OF OVULES- 

 FERTILIZATION AND FORMATION OF SEEDS. 



Pollination is followed by fertilization, which con- 

 sists in the union and complete fusion of the male cell 

 with the female cell. The male cell is the pollen- 



