that it does explode, after a time, spontaneously, without the excitement of a touch. In one large spike, every flower had been visited by insects, and all the pollinia had been removed. Another unusually fine spike from South Kent, sent me by Mr. Oxenden, had borne forty-one flowers, and produced twenty-seven large seed-capsules, besides some smaller ones.
The pollen resembles that of Listera, in consisting of grains (formed of four granules) tied together by a few weak threads; it differs in being much more incoherent, so that after a few days it swells and overhangs the sides and summit of the rostellum: hence, if the rostellum of a rather old flower be touched, and an explosion be caused, the pollinia are not so neatly caught by their tips as in Listera; consequently a good deal of the friable pollen is often left behind in the anther-cells and is apparently wasted. Portions of this fall on the corolla; and as the pollen in this state readily adheres to any object, it is not improbable that insects crawling about would thus get dusted, and leave some on the viscid stigma, without having touched the rostellum and caused it to explode. Certainly if the labellum were more upturned, so that insects were forced to brush against the anther and column, they would get smeared with the pollen as soon as it had become friable, and might thus effectually fertilise the flower.
This observation interested me, because, when previously examining Cephalanthera, with its aborted rostellum, its upturned labellum, and its friable pollen, I had speculated how a transition, with each gradation useful to the plant, could possibly have been effected from the state of the pollen and flower in the allied Epipactis, with its pollinia attached to a well-developed rostellum, to the present condition of Cephalanthera. Neottia nidus-avis shows to a certain extend how such a transition might have been effected. This Orchid is at present mainly fertilised by means of the explosive rostellum, which acts effectually only as long as the pollen remains in mass; but unless we suppose that the circumstance of the pollen soon becoming friable is a mere injury to the plant, we may believe that the pollen in this condition is sometimes transported to the stigma by its adhesion to the hairy bodies of insects. If this be so, we can see that a slight change in the form of the flower, and by the pollen becoming friable at a still earlier age, this means of fertilisation might be rendered more and more effectual, and the explosive rostellum less and less useful. Ultimately the rostellum would become a superfluity; and then, on the great principle of the economy of organisation, rendered so necessary by the struggle for live, by which every part of every being tends to be saved, the rostellum would be absorbed or aborted. In this case we should have the production of a new Orchid in the condition of Cephalanthera, as far as its means of fertilisation are concerned, but in general structure still closely allied to Neottia and Listera.
CHAPTER V.
Cattleya, simple manner of fertilisation—Masdevallia, curious closed flower—Dendrobium, contrivance for self-fertilisation—Vandeæ, diversified structure of the pollinia; importance of the elasticity of the pedicel; its power of movement—Elasticity and strength of the caudicle—Calanthe with lateral stigmas, manner of fertilisation—Angræcum sesquipedale, wonderful length of nectary—Acropera, perplexing case, a male Orchid.
HAVING examined the means of fertilisation in so many British Orchids, belonging to fourteen genera, I was anxious to ascertain whether the exotic forms, belonging to quite distinct Tribes, equally required insect-agency. I especially wished to ascertain whether rule holds that each flower is necessarily fertilised by pollen brought from a distinct flower; and in a secondary degree I was curious to know whether the pollinia underwent those curious movements of depression by which they are placed, after transportal by insects, in the proper position to strike the stigmatic surface.
By the kindness of many friends and strangers I have been enabled to examine fresh flowers of several species, belonging to forty-three exotic genera, well dispersed through the sub-families of the vast Orchidean series.[1] It is not my intention to describe the means of fertilisation in all these genera, but merely to pick out a few curious cases, and other cases which illustrate the foregoing descriptions. The diversity of the contrivances, almost all adapted to favour the intercrossing of distinct flowers, seems to be exhaustless.
Mr. James Veitch, jun., has generously given me many beautiful Orchids, some of which were of especial service. Mr. R. Parker also sent me an extremely valuable series of forms. Lady Dorothy Nevill most kindly placed her magnificent collection of Orchids at my disposal. Mr. Rucker of West Hill, Wandsworth, sent me
- ↑ I am particularly indebted to Dr. Hooker, who on every occasion has given me his invaluable advice, and has never become weary of sending me specimens from the Royal Gardens at Kew.
