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.[1]
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We thus see how beautifully everything is contrived, that the pollinia should be withdrawn by insects visiting the flowers.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
- ↑ Professor Asa Gray was so kind as to examine for me Spiranthes gracilis and cernua in the United States. He found the same general structure as in our S. autumnalis, and was struck with the narrowness of the passage into the flower. He has since confirmed ('Amer. Journ. of Science,' vol. xxxiv. p. 427) my account of the structure and action of all the parts in Spiranthes, with the exception that it is the column and not the labellum, as I formerly thought, which moves as the flowers become mature. He adds that the widening of the passage, which plays so important a part in the fertilisation of the flower, "is so striking that we wonder how we overlooked it."
