AN ENCYCLOPAEDIA OF HORTICULTURE, 



517 



Orchid Fertilisation continued. 



in the suitable position to reach the stigmas of the next 

 plant visited. The long, narrow, spurless labellum, 

 secretes honey into a groove down the middle of the 

 npper surface. The insect follows up this guide as 

 it climbs up the labellum, and, on reaching the upper 

 end of the groove, it is almost certain to touch the 

 rostellum, and to have the pollen-masses affixed to its 

 head. For a little the rostellum sinks, and closes the 

 way to the stigmas ; but soon it rises, leaving free access 

 to the stigmas for later visitors to the flower. These 

 visitors are attracted by a new supply of nectar in the 

 groove. They are almost all small insects, with the 

 proboscis short or absent, the nectar lying open. The 

 genus Cypripedium represented by C. Cal- 

 ceolus, the " Lady's Slipper " (see Fig. 763), 

 a scarce species in England, and by many 

 tropical species, e.g., C. spectabile (see Fig. 764) 

 belongs to a group that possess two fertile 

 stamens, situated at each side, on the lower 

 surface, of a broad, shield-like plate (the re- 

 presentative of the single fertile stamen in 

 other Orchids). This plate overhangs the 

 fitigmatic surface, which lies on a prominent 

 overhanging rostellum immediately below the 

 plate. The labellum is much like the front 

 half of a slipper (see Fig. 764). It forms a 

 chamber, open above, and large enough to 

 allow small bees to enter. This they do 

 readily, to gnaw the hairs that line the middle 

 of the floor. The lips on each side are in- 

 curved, so that, when an insect wishes to 

 escape by the opening, it cannot crawl out as 

 it entered ; nor is there room for it to fly 

 out. But, at the stalk of the labellum, a small 

 opening is left on each side, where, if strong 

 enough, by squeezing between the labellum 

 and the stigma, and then between the labellum 

 and either stamen, the prisoner can escape, 

 carrying away one of the pollinia. As the 

 fitigma is touched before the anther, the flower 

 cannot be fertilised with its own pollen, but 

 only with that from one previously visited. 

 Few flowers are formed on each plant. 



Among exotic Orchids, the adaptations for 

 fertilisation by pollen from other flowers are 

 far more curious and striking even than the 

 above, and the peculiarities in form and struc- 

 ture of the rostellum, the pollinia, and other 

 parts of the flower, are often very strange, and 

 cause them to resemble insects (e.g., in the Fly 

 -Orchis), or a minute dove, or other objects 

 too numerous to describe, or even to mention. 

 Mr. Darwin has described fully some of the 

 more remarkable, among which is the extremely curious 

 genus Catasetum. In this genus of Orchids, the flowers 

 are of different sexes, and they are so unlike that on 

 C. tridentatum three supposed genera were based; the 

 characters being drawn from the form of the flowers. 

 These were Catasetum, based on the male ; Myanthus, on 

 the hermaphrodite ; and Monachanthus, on the female 

 flowers. The occasional presence of these flowers on 

 the same plants has enabled botanists to understand 

 their mutual relations. Cross-fertilisation must be 

 effected before seeds can be formed ; and this requires 

 insect agency. In Catasetum (the male flower), the 

 labellum rises up at the back of the flower (the ovary 

 not making the usual half-twist), and is shaped like a 

 monk's cowl. It contains no exuded nectar, but bees 

 and other insects gnaw the inner surface greedily. The 

 central column projects forward below the labellum, 

 and bears the anther on its upper surface, the polliuia 

 and disk being embedded in the tissues till set free 

 ,by the mechanism now to be described. From each 

 .side of the rostellum grows a long, slender, curved horn, 



Orchid Fertilisation continued. 



or " antenna " ; and both lie within the hollow of tho 

 labellum. The right antenna occupies the bottom of 

 the space, with its tip just projecting beyond the left 

 margin; the left one curves upwards along the back of 

 the labellum. 



The slightest touch on either antenna immediately 

 transmits some stimulus to the membrane over the disk 

 of the pollinia, causing it to be rent; the disk springs 

 out, by sudden removal of tension from the bent 

 caudicle, and the whole mass is thrown forward, with 

 the sticky disk in front, against the insect, if the 

 antenna has been touched by an insect, and at once 

 adheres in the position most likely to aecnre convey- 



FIG, 765. FLOWER OF CATTLEYA BICOLOII. 



ance to the stigma of the female flower. The pollinia 

 are not sensitive to contact of insects with any part 

 of the flower except the antennae; but the position of 

 these organs renders this contact almost certain to 

 occur when the labellum is being gnawed. The female 

 flowers of this species -(formerly called Monachanthus 

 viridis), resemble the male flowers in the position and 

 general form of the labellum, which differs only in 

 minor details. The column also agrees in position, but 

 is smaller; the pollinia are rudimentary; the disk does 

 not cohere with the pollinia, but soon falls away: and 

 the antennae are absent. On the other hand, the stigmas 

 and the ovaries are developed, and the seeds ripen 

 abundantly. 



The form called Myanthus barbatus is very different 

 in appearance from the other two, and Darwin points out 

 that it comes near, in its structure, to the flowers of 

 Catasetum callosum and of C. saccatum, but that it is 

 hermaphrodite, though not known to seed. He suggests 

 that it may be a reversion to the ancestral form. Owing 

 to a bend in the ovary, the column and the long-fringed 



