Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/48

The allied Masdevallia fenestrata is an extraordinary flower; for the three sepals, instead of closing, as in Stelis after the flower has remained for a time expanded, always cohere together and never open. Two minute, lateral, oval windows (hence the name fenestrata), seated high up the flower and opposite each other, afford the only entrance into the flower; but the presence of these two minute windows (Fig. XX.) shows how necessary it is that insects should have access in this case as with all other Orchids. How insects perform the act of fertilisation I have failed to understand. At the bottom of the roomy and dark chamber formed by the closed sepals, the minute column is placed, in front of which the furrowed labellure stands, with a highly flexible hinge, and on each side the two upper petals; a little tube being thus formed. Hence, when a minute insect enters, or a larger insect inserts its proboscis through either window, it has by touch to find the inner tube in order to reach the curious nectary at its base. Within this little tube, formed by the column, labellure and lateral petals, a very broad and hinged rostellum projects at right angles, the under surface of which is viscid; the minute caudicles of the pollinia, projecting out of the anther-case, rest on the base of the upper membranous surface of the rostellum. The stigmatic cavity is deep. After cutting away the sepals I vainly endeavoured, by pushing a bristle into the tubular flower, to remove the pollinia. The whole structure of the flower seemed carefully intended to prevent the withdrawal of the pollinia, as well as their subsequent insertion into the stigmatic chamber! Some new and curious contrivance has here to be made out.

Of Bolbophyllum I examined the curious little flowers of four species, which I will not attempt fully to describe. In B. cupreum and cocoinure, the upper and lower surfaces of the rostellum resolve themselves into viscid matter, which has to be forced upwards by the insects into the anther, so as to secure the pollinia. I effected this easily by passing a needle down and withdrawing it from the flower, which is rendered tubular by the position of the labellure. In B. rhizophone the anther-case moves back, when the flower is mature, and leaves the two pollen-masses fully exposed and adhering spontaneously to the upper surface of the rostellum. The two pollen-masses adhere together by viscid matter, and, judging from the action of the bristle, are always removed together. The stigmatic chamber is very deep, and its orifice is oval, and is exactly fitted by one of the two pollen-masses. After the flower has remained open for some time, the sides of the oval orifice of the stigmatic chamber close in and shut it completely, a fact which I have observed in no other Orchid, and which, I presume, is here related to the much exposed condition of the whole flower. When the two pollinia were attached to a needle or bristle, or were forced against the stigmatic chamber, one of the two masses glided into the small orifice more readily than could have been anticipated. Nevertheless it is evident that insects must place themselves on successive visits in precisely the same position, so as first to remove the two pollinia, and then force one of them into the stigmatic orifice. The two upper filiform petals would serve as guides to the insect; but the labellure, instead of making the flower tubular, hangs just like a tongue out of a widely open mouth.

The labellum in all the species which I have seen, more especially in B. rhizophoræ, is remarkable by being joined to the base of the column by a very narrow, thin, white strap, which is highly elastic and flexible; it is even highly elastic when stretched, like an india-rubber band. When the flowers of this latter species were blown by a breath of wind the tongue-like labellures all waggled about in a very odd manner. In some species not seen by me, as in B. barbigerum, the labellure is furnished with a beard of fine hairs, and these cause the labellure to be in almost constant motion from every breath of air. What the use can be of this extreme flexibility and liability to movement in the labellure, I cannot conjecture, unless it be to attract the notice of insects to their dull-coloured, small, and inconspicuous flowers, in the same manner as the bright colours and strong odours of many other Orchids apparently serve to attract insects.

Of the many singular properties of Orchids, the irritability of the labellum in several distantly allied forms is highly remarkable. When touched, it is described as quickly moving. This is the case with some of the species of Bolbophyllum, but I could not detect any irritability in the species which I examined; nor have I, much to my regret, seen any Orchid with an irritable labellum. The Australian genus Calæna is endowed with this property in a most remarkable degree; for when an insects alights on its labellum it suddenly shuts up against the column, and encloses its prey as it were in a box. Dr. Hooker^[1] believes that this action aids in some manner in the fertilisation of the plant.

1. 'Flora of Tasmania,'vol. ii. P. 17, under Calæna.