within the labellum and projecting a little beyond the column. The grains of pollen are immersed in, and coated by, viscid fluid, which is so glutinous that it can be drawn out into threads. As the two anthers stand behind and above the lower convex surface (see Fig. B) of the stigma, it is impossible that the glutinous pollen can get on to this, the fertile surface, without mechanical aid.
An insect could reach the extremity of the labellum, or the toe of the slipper, through the longitudinal dorsal slit; but according to all analogy the basal portion in front of the stigma would be the most attractive part. Now, as the flower is closed at the end, owing to the toe of the labellum being upturned, and as the dorsal surface of the stigma, together with the large shield-like rudimentary anther, almost close the basal part of the medial slit, two convenient passages alone are left for an insect to reach with its proboscis the lower part of the labellum; namely, directly over and close outside the two lateral anthers. If an insect where thus to act, and it could hardly act in any other way, it would infallibly get its proboscis smeared with the glutinous pollen, as I found to occur with a bristle thus inserted. When the bristle smeared with pollen was pushed further into the flower, especially if pushed in by the little notch outside the anther, some of the glutinous pollen was generally left on the slightly convex stigmatic surface. The proboscis of an insect would effect this latter operation better than a bristle, owing to its flexibility and power of movement. Thus an insect would place either the flower's own pollen on to the stigma, or, flying away, would carry the pollen to another flower. Which of these two contingencies commonly occurs, will depend on whether the insect first inserts its proboscis directly over the anther, or outside by the little notch.
We thus see how important, or rather how necessary for the fertilisation of the plant, is the curious slipper-like shape of the labellum, in leading insects to insert their probosces by the lateral passages close to the anthers. The upper, shield-like, rudimentary anther is equally, and in the same manner, necessary.
The economy shown by nature in her resources is striking: in all Orchids seen by me, excepting Cypripedium, the stigma is more or less concave and is viscid, so as to secure the dry pollen, brought to it by means of the viscid matter secreted by the modified stigma, called the rostellum. In Cypripedium alone the pollen is glutinous, and takes on itself the function of viscidity, which in other Orchids belongs to both the true stigma and the modified stigma or rostellum. The stigma itself, on the other hand, in Cypripedium entirely loses its viscidity, and at the same time becomes slightly convex, so as more effectually to rub off the glutinous pollen adhering to an insect's proboscis. Thus the act of fertilisation is completed, and there is no superfluity in the means employed.
SECRETION OF NECTAR.
Many exotic Orchids secrete plenty of nectar in our hot-houses. I have found the horn-like nectaries of Aerides filled with fluid; and Mr. Rodgers, of Sevenoaks, informs me that he has taken crystals of sugar of considerable size from the nectary of A. cornutum. In nearly all the flowers of Angrecum distichum sent me from Kew, insects had bitten holes through the nectaries, so as to get more readily at the nectar: if insects were invariably to follow this bad habit in the plant's native African home, undoubtedly it would soon become extinct, for it would never produce a seed. The nectar-secreting organs present great diversity in structure and position in the various genera; but apparently always form part of the base of the labellum. In Dendrobium chrysanthum the nectary consists of a shallow saucer; in Evelyna of two large united cellular balls; in Bolbophyllum cupreum of a medial furrow. In Cattleya the nectary penetrates the ovarium; in Angrecum sesquipedale it attains, as we have seen, the astonishing length of above eleven inches; but I will not detail the several cases. The nectar-secreting apparatus of Coryanthes, however, is so remarkable, as described by M. Ménière,[1] that it must not be quite passed over: two little horns near the strap-like junction of the labellum with the base of the column, secrete so much limpid nectar, with a slightly sweet taste, that it slowly drops down. M. Ménière estimates the total quantity secreted by a single flower at about an English ounce. But the remarkable point is that the deeply-hollowed end of the labellum hangs some way down, exactly beneath the two little horns, and catches the drops as they fall, just like a bucket suspended some way beneath a dripping spring of water.
Although the secretion of nectar is of the highest importance to Orchids by attracting insects, which are indispensable to their fertilisation, yet it would seem that the secretion acts also, at least in some cases, as an excretion. One may suspect this to be the case with the Coryanthes from the immense quantity secreted; but the bractee of some Orchids have been observed[2] to secrete nectar; and as these lie outside the flower, the secretion cannot serve any useful purpose by attracting insects. Mr. Rodgers informs me that he has observed much nectar secreted at the base of the footstalks of the flowers in Vanilla. It is in perfect accordance with
- ↑ 'Bulletin de la Soc. Bot. de France,' tom ii. 1855, p. 351.
- ↑ J. G. Kurr, 'Ueber die Bedeutung der Nektarien,' 1833, s. 28, on the authority of Treviranus and Curt. Sprengel. The calyx of certain species of Iris also (idem, s. 25) secretes nectar. I have observed much nectar secreted by the stipules of Vicia sativa and faba, which is eagerly collected by bees. Glands on the under side of the leaves of the common laurel also secrete nectar, which, though the drops are extremely minute, is sought for by various insects.
