Chap. IV. SPIRANTHES 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 pollinia 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 that the pollinia should be withdrawn by insects visit- 

 in o- 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 



♦ Profeasor Asa Gray was so of all the parts in Spirantlies with 



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



ranthes qradUs and cernua in the atid not the labellum, as I formrr- 



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



same general structure as in our flowers becomematu-e^ He adds 



8. autumnalis, and was struck that the widening oi the passage, 



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



into the flower. He hns since in the fe.tihsation of the flower, 



confirmed ('Amer. Journ. of " is so striking t^iat we wundei 



Science/ vol. xxxiv. p. 427) my how we overlooked it. 

 account of the structure and action 



