to say. In the case of these latter flowers, it is of some importance to note that not a grain of pollen had got on their stigmas (and I looked carefully), and their ovaria had not swollen. These several facts clearly show that the removal of the pollinia by insect-agency is absolutely necessary to the fertilisation of this species.
That insects do their work effectually, the following cases show: A rather young spike, with many buds in the upper part, had the pollinia unremoved from the seven upper flowers, but they had been completely removed from the ten lower flowers: there was pollen on the stigmas of six of these ten lower flowers. In two spikes taken together, the twenty-seven lower flowers had their pollinia removed, with pollen on the stigmas of all; these were succeeded by five open flowers with pollinia not removed, and with no pollen on their stigmas; and these were succeeded by eighteen buds. Lastly, in an older spike with forty-four flowers, all fully expanded, the pollinia had been removed from every one; and on all the stigmas that I examined there was pollen, generally in large quantity.
It will perhaps be worth while to recapitulate the several special adaptations for the fertilisation of this Orchid. The anther-cells open early, leaving the pollen-masses quite loose, with their tips resting on the concave crest of the rostellum. The rostellum then slowly curves over the stigmatic surface, so that its explosive crest stands at a little distance from the anther; and this is very necessary, otherwise the anther would be caught by the viscid matter, and the pollen for ever locked up. This curvature of the rostellum over the stigma and base of the labellum is excellently well adapted to favour an insect striking the crest when it raises its head, after having crawled up the labellum, and licked up the last drop of nectar at its base. The labellum, as C. K. Sprengel has remarked, becomes narrower where it joins the column beneath the rostellum, so that there is no risk of the insect going too much to either side. The crest of the rostellum is so exquisitely sensitive, that a touch from a minute insect causes it to rupture at two points, and instantaneously two drops of viscid fluid are expelled, which coalesce. This viscid fluid sets hard in so wonderfully rapid a manner that it rarely fails to cement the tips of the pollinia, nicely laid on the crest of the rostellum, to the insect's forehead. As soon as the rostellum has exploded it suddenly curves downward till it projects at right angles over the stigma, protecting it in its early state from impregnation, in the same manner as the stigma of Spiranthes is protected by the labellum clasping the column. But as in Spiranthes the labellum after a time moves from the column, leaving a free passage for the introduction of the pollinia, so here the rostellum moves back, and not only recovers its former arched position, but stands upright, leaving the stigmatic surface, now become more viscid, perfectly free for pollen to be left on it. The pollen-masses, when once cemented to an insect's forehead, will generally remain firmly attached to it until the viscid stigma of a mature flower remove these encumbrances from the insect, by rupturing the weak elastic threads by which the grains are tied together receiving at the same time the benefit of fertilisation.
Listera cotdata
Professor Dickie of Aberdeen was so kind as to send me two sets of specimens, but I applied rather too late in the season. The structure is essentially the same as in the last species, and the loculi of the rostellum were very plain. In the middle, on the crest of the rostellum, two or three little hairy points project; whether these have any functional importance I know not. The labellum has two basal lobes (of which vestiges may be seen in L. ovata) which curve up on each side, and would compel an insect to approach the rostellum straight in front. Two flowers had been touched either during the journey, or too quickly by some insect, and had exploded; and their pollinia in consequence were firmly cemented to the crest of the rostellum; but in most of the spikes all the pollen-masses had been renoved by insects.
Neottia nidus-avis
I made numerous observations on this, the Bird's-nest Orchis,[1] but they are not worth giving, as the action and structure of every part is almost identically the same as in Listera ovata. The labellum secretes plenty of nectar, which I mention merely as a caution, because during one cold and wet season I looked several times and could not see a drop, and was perplexed at the apparent absence of any attraction for insects; nevertheless, had I looked more perseveringly, I should probably have found nectar.
Whether the rostellum ultimately explodes, if not touched, I could not ascertain; that it long remained unexploded, though ready to act, is certain; but I found in 1860 so many flowers exploded, with a bead of the purplish hardened cement attached to the crest of the rostellum and to the unremoved pollinia, that I suspect
- ↑ This unnatural sickly-looking plant has generally been supposed to be parasitic on the roots of the trees under the shade of which it lives; but, according to Irmisch ('Belträge zur Biologie und Morphologie der Orchideen,' 1853, s. 25), this certainly is not the case.
