224 NATIVE BRITISH ORCHIDACE^ 



O. muscifera about sLx minutes according to Darwin).^ As the insect usually spends 

 some time on the flower, this prevents pollination of another flower on the same spike, 

 for until the downward movement is completed the pollinia cannot touch the stigma. 

 The pollinia then point horizontally forward, but the slight bend or elbow in the 

 caudicle causes the two pollen-masses to incline slightly upwards, so that they are 

 in exactly the right position to come in contact with the stigma, wliich is on the imier 

 surface of the ceiling and walls of the little arched chamber formed by the base of 

 the column. It is easily distinguished by its glistening surface, due to the viscid 

 secretion, and is often bordered by a coloured line. The secretion is sufficiently 

 tenacious to detach packets of pollen from the pollinia. Stimulated by the stigmatic 

 fluid, these disintegrate into their component pollen-grains, which put forth pollen- 

 tubes. These grow down like roots in a seed-bed to the ovary, each tube entering 

 an ovule (immature seed) by the micropyle, and discharging its life-giving contents, 

 without wliich the ovule could never develop into a fertile seed. 



The rostellum consists of the two small cup-shaped pouches, each enclosing a viscid 

 disc. In Orchis both discs are enclosed in the same pouch and, on the withdrawal 

 of the insect, the pouch moves back to its former position, so that if only one vis- 

 cidium is removed, the other is again immersed in liquid and kept from setting hard. 

 As the pouch in Ophrys only holds one poUinium, it does not need to resume its 

 position, and does not do so. In Orclns the stigma is vertical, and the straight pollinia 

 only need to point horizontally forwards to come in contact with it. In Ophrys it is 

 partly on the ceiling of the little chamber. The tips of the pollinia must therefore 

 point upwards — hence the beautiful adjustment of an elbowed stalk, which is of the 

 greatest service to the plant. If the pollinia are withdrawn on a pencil, it will be 

 seen that the elbow finally rests on the pencil, tilting the pollinia slightly upwards. 



The European Orchidaceas attain their highest stage of evolution in the genus 

 Ophrys. The high degree attained by Orchis is surpassed by the provision of a separate 

 pouch for each of the two viscid discs instead of a common pouch to hold both, 

 and by the substitution of sexual allurement for that of edible matter. Having no 

 honey, the flowers are shunned not only by insects generally, but even by the females 

 of the particular species by the males of which the flowers are pollinated. Certain 

 species of Ophrys are so highly specialised that they mimic the females of one par- 

 ticular species of insect, and are solely visited by the males of that species. To this 

 category belong Ophrys speculum of N. Africa, and probably also our 0. muscifera, 

 which, as far as is at present known, is only visited by Gorytes mystaceus. 



' Fert. Orch. ed. 2, p. 46. 



