THE COLOURS OF ANIMALS AND PLANTS 145 



other stigmas and the entrance to the nectary. The 

 pollinia, which lie in the anther cells, are club-shaped, 

 the head of the club consisting of a number of 

 packets of pollen grains united by thin elastic 

 threads : the stalk of the club ends in a disc with a 

 ball of very viscid matter on its under side, lying in 

 the rostellum. The anther cells open when ripe, 

 exposing the pollinia ; the rostellum is very delicate, 

 and is ruptured by the slightest touch, exposing the 

 viscid balls. 



The problem is to transfer the pollinia from one 

 spike of flowers to another. The manner in which 

 this problem is solved through the agency of insects 

 is as follows. 



The insect, alighting on the labellum, pushes its 

 head into the flower in order to reach the spur with 

 its proboscis. In doing this it knocks against the 

 rostellum, displacing its covering membrane and 

 exposing the viscid balls to which the pollen masses 

 are attached. On withdrawing its head the pollen 

 masses come away firmly cemented to it and stand- 

 ing erect. In about thirty seconds the viscid disc 

 contracts, causing the pollen mass to bend forwards 

 through an angle of 90 , so as to become horizontal. 

 By this contraction the pollen mass will be in a 

 position to be applied directly to the stigma, when 

 the insect visits the next flower. This manoeuvre 

 can be imitated by pushing the point of a pencil 

 into a flower as shown in the figure, when the 

 pollen masses will come away fixed to the pencil. 

 (Fig. 27.) 



In this way an insect flying from flower to flower 



K 



