516 



THE DICTIONARY OF GARDENING, 



Orchid Fertilisation continued. 



balls lie almost side by side, and are covered by an 

 elastic membrane formed by the rostellum. This mem- 

 brane prevents the disks from drying, which would 

 render them useless. It is at first continuous over 

 the disks, but it soou tears across, and, on being touched, 

 it is easily dragged down, leaving them bare. 



When a suitable insect (e.g., a moth) visits the flowers, 

 and conveys the pollen of one flower to the stigmas 

 of the next one visited, it pushes the proboscis down 

 the tube of the labellum, and, in doing so, its pro- 

 boscis, or its head, rubs against the membrane, and, 

 depressing it, lays bare the sticky disks of the pollinia. 

 These, as soon as they are touched, adhere to the part 

 with which they are in contact, and, on the insect with- 

 drawing its head, one or both of the pollinia are 

 also withdrawn. They are erect when first withdrawn, 

 but, in about half a minute, as the disks dry, the 

 pollinia bend forward, and thus come to lie nearly i 

 alongside the proboscis, or other body, in the very | 



Orchid Fertilisation continued. 



aid in transferring the pollen is not given. There is no 

 means in this plant of conveying pollen to the stigma, 

 without extraneous help, and the adaptations for cross- 

 fertilisation are complete. 



It may, therefore, be regarded as the type of a large 

 section of Orchids, in so far as concerns its entire depen- 

 dence on extraneous aid, though, in mine* points, there is 



FIG. 763. CVPRIPEDIUM CALCEOLUS. 



best position to bring them into contact with the stigmas 

 of flowers visited later. Owing to the little interval 

 that elapses before the pollinium is in the suitable 

 position, the insect has usually visited all the flowers on 

 a stem before any of the pollen could be left in flowers 

 of the plant from which it was taken; hence, cross-fer- 



Hisation between different plants is favoured. When a 



Umum touches a stigma, some of the small masses 



adhere, and are dragged off, and thus one pollinium 



suffices for several stigmas. Insects have frequently 



>een taken bearing pollinia, and often several pollinia 

 inject adherin & to the proboscis and head of an 



0. maculata, like most of this family, remains barren 

 if insects are prevented from visiting the flowers, and 



FIG. 764. SINGLE FLOWER OP CYPRIPEPIUM SPECTABII.E. 



a considerable diversity in the methods of favouring cross- 

 fertilisation ; e.g., in 0. pyramidalis, the sticky disks 

 are united into a concave band, which adheres to the 

 proboscis of the insect visitors. The pollinia pass through 

 the same change of position as in O. maculata. 



Listera ovata, the " Twayblade," which is an incon- 

 spicuous, gi-eenish Orchid, growing in grassy places, 

 has the pollen masses lying immediately above the ros- 

 tellum. The pollen masses are dry, and, therefore, cannot 

 adhere of themselves to insects ; but when the tip of the 

 rostellum is touched, even very gently, by the insect, a 

 drop of sticky fluid oozes out, and fixes the pollen grains 



