LISTERA 105 



sisting in the stalk after the blossoms have withered, "helps 

 to define the genus. 



Our illustration (Plate XLII.) shows the parts that 

 Darwin studied before he fully learned the mechanism of 

 cross fertilisation of the English species of Listera ovata. 

 Fig. I is a blossom of Listera Australis. 



All of the petals except the split lip have been removed 

 (Fig 2), leaving the ribbed ovary and the internal structure of 

 the column. Under the anther (a), with its pollen masses 

 (/)), lies the stigma surface (s), with the rostellum projecting 

 out like a thin leaf between. The rostellum is slightly 

 hollow on its upper surface, and is extraordinarily irritable, 

 for no matter how slightly it is touched, with a needle point 

 or a human hair, or even influenced by the vapour of chloro- 

 form, a large drop of viscid fluid is instantly expelled. 



The pointed tips of the pollen masses lie immediately 

 over the rostellum, tied together by a few weak elastic v 

 threads that are easily broken. 



Darwin observed that the agent that jogged the rostellum, 

 whether artificial needle tip or head of insect, never failed to 

 receive the drop of sticky exudation and the pollen mass at 

 the same time. He noticed that even when a bunch of 

 flowers was carried home in the hand, the sepals or petals 

 that accidentally brushed against the rostellum caused it to 

 explode its sticky drop and glue the pollen mass to the 

 fluttering petals, so that on reaching home it looked as 

 though the flowers had been shooting out their pollen to a 

 distance. He also noticed that after the pollen masses had 



