Chap. IV. SriEANTHES AUTUMNALIS. Ill 



passage is left for insects to deposit pollen on the 

 stigmatic surface. On this slight movement of the 

 column the fertilisation of the flower absolutely 

 depends.* 



With most Orchids the flowers remain open for 

 some time before they are visited by insects; but 

 with Spiranthes I have generally found the boat- 

 formed discs removed very soon after their expansion. 

 For example, in the two last spikes which I happened 

 to examine there were numerous buds on the summit 

 of one, with only the seven lowest flowers expanded, 

 of which six had their discs and poUinia removed; 

 the other spike had eight expanded flowers, and the 

 pollinia of all were removed. We have seen that when 

 the flowers first open they would be attractive to 

 insects, for the receptacle already contains nectar; 

 and at this period the rostellum lies so close to the 

 channelled labellum that a bee could not pass down 

 its proboscis without touching the medial furrow of 

 the rostellum. This I know to be the case by repeated 

 trials with a bristle. 



We thus see how beautifully everything is contrived 

 that the pollinia should be withdrawn by insects visit- 

 ing the flowers. They are already attached to the 

 disc by their threads, and, from the early withering 

 of the anther-cells, they hang loosely suspended but 

 protected within the clinandrum. The touch of the 



* Profeaaor Asa Gray was so of all the parts in Spiranthes, with 

 kind as to examine for me Spi- the exception that it is the column 

 ranthes gracilis and cerntm in the and not the labellum, as I former- 

 United States. He found the ly thought, which moves as the 

 same general stnicture as in our flowers become matu'e. He adds 

 S. autumnalis, and was struck tliat the widening ol the passage, 

 with the narrowness of the passijge which plays so important a part 

 into the flower. He has since in the fertilisation of the flower, 

 confirmed (' Amer. Jouni. of " is so striking that we wondei 

 Science,' vol. xxxiv. p. 427) my how we overlooked it." 

 account of the structure and action 



