When the flower first opens the receptacle contains nectar, and at this period the front of the rostellum, which is slightly furrowed, lies close to the channelled labellum; consequently a passage is left, but so narrow that only a fine bristle can be passed down it.[1] In a day or two the labellum moves a little farther from the rostellum, and a wider passage is left to the stigmatic surface. On this slight movement of the labellum 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 disc removed very soon after the expansion of the flower. For example, in the two last spikes which I happened to examine there were in one numerous buds on the summit, with the seven lowest flowers alone expanded, of which six had their discs and pollinia removed; the other spike had eigth expanded flowers, with the pollinia of all removed. We have seen that when the flower first opens it would be attractive to insects, for the receptacle already contains nectar; and the rostellum at this period lies so close to the channelled labellum that a bee or moth 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.
Let it be observed how beautifully everything is contrived that the pollinia should be withdrawn by an insect visiting the flower. The pollinia 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 proboscis causes the rostellum to split in front and behind, and frees the long, narrow, boat-formed disc, which is loaded with extremely viscid matter, sure to adhere longitudinally to the proboscis. When the bee flies away, so surely will it carry away the pollinia. As the pollinia are attached parallel to the disc, they will become attached parallel to the proboscis. Here, however, appears a difficulty; when the flower first opens and is best adapted for the removal of the pollinia, the labellum lies so close to the rostellum, that the pollinia, when attached to the proboscis of an insect, could not possibly be forced into the flower so as to reach the stigma; they would be either upturned or broken off; but we have seen that after two or three days the labellum becomes more reflexed and moves from the rostellum,—a wider passage being thus left. When in this condition, I have tried with the pollinia attached to a fine bristle; and by inserting the bristle into the nectar-receptacle (n, Fig. B), it is pretty to see how sheets of pollen are left adhering to the viscid stigma. It may be observed in the diagram, B, that the orifice into the nectar-receptacle lies, owing to the projection of the stigma, close to the lower side of the flower; hence insects would insert their probosces on this side, and an open space is thus afforded for the attached pollinia to be carried down to the stigma, without being brushed off. The stigma evidently projects so that the ends of the pollinia may strike against it.
Hence, in Spiranthes, not only the pollen must be carried from one flower to another, as in most Orchids, but a lately expanded flower, which has its pollinia in the best state for removal, cannot then be fertilised. Generally old flowers will be fertilised by the pollen of younger flowers, borne, as we shall see, on a separate plant. In conformity with this I observed that the stigmatic surfaces of the older flowers were far more viscid than those of the younger flowers. Nevertheless, a flower which in its early state had not been visited by insects would not necessarily, in its later and more expanded condition, have its pollen wasted; for insects, in inserting and withdrawing their probosces, bow them forwards, and would thus often strike the furrow of the rostellum. I imitated this action with a bristle, and often succeeded in withdrawing the pollinia: I was led to try this from at first choosing older flowers for examination; and passing a bristle, or fine culm of grass, straight down into the nectary, the pollinia were never withdrawn; but when I bowed it forward, I succeeded. Those flowers which have not their own pollinia removed can of course be fertilised, and I have seen not a few cases of flowers with their pollinia still in place, with sheets of pollen on their stigmas.
At Torquay I watched a number of these flowers growing together for about half an hour, and saw three humble-bees of two kinds visit them. I caught one and examined its proboscis: on the upper lamina, some little way from the tip, two perfect pollinia were attached, and three other boat-formed discs without pollen; so that this bee had removed the pollinia from five flowers, and had probably left the pollen of three of them on the stigmas of other flowers. The next day I watched the same flowers for a quarter of an hour, and caught another humble-bee at work; one perfect pollinium and four boat-formed discs adhered to its proboscis, one on the top of the other, showing how exactly the same part had each time touched the rostellum.
The bees always alighted at the bottom of the spike, and, crawling spirally up it, sucked one flower after the other. I believe humble-bees generally act thus when visiting a dense spike of flowers, as it is most convenient for them; in the same manner as a woodpecker always climbs up a tree in search of insects. This
- ↑ Professor Asa Gray was so kind as to examine Spiranthes gracilis and cernua in the United States. He found the same general structure as in our S. autumnalis, and he was struck with the narrowness of the passage into the flower.
