II ORCHIDACE^ 411 



situated just above the stigma ; and while the flower is 

 in bud, or at any rate before it becomes quite open, the 

 anther opens and expels the pollen, which stands in two 

 almost free columns, resting on the sharp edge of the 

 stigma, and emits a number of tubes which deeply 

 penetrate the stigmatic tissue. These serve partially, 

 but, as Darwin has shown, only partially, to fertilise the 

 flower ; he suggests that the principal use of this closing 

 of the flower and emission of the pollen tubes is probably 

 to support the pollen, which would otherwise fall out of 

 the flower. In this curious manner, however, they are 

 retained in a proper position until the flower is visited 

 by insects, to which they readily adhere ; and which are 

 necessary to ensure the perfect fertility of the plant. 

 Darwin found that plants which were covered up only 

 ripened one-seventh of the usual quantity of seeds. 



0. ensifolia. — According to Delpino this species is 

 always fertilised by insects. While sucking the nectar, 

 or as the proboscis is drawn back, the insect slightly 

 smears itself with the sticky fluid which clothes the 

 stigma, and then pressing the anther, carries off' some 

 of the pollen, which it deposits on the stigma of the 

 next flower it visits. 



LiSTERA (Twayblade) 



A raceme of small greenish flowers, over a pair of 

 leaves, which in L. ovata are 2-4 inches long, in 

 L. cor data less than an inch. 



L. ovata. — In the centre of the lip is a narrow nectar- 

 secreting tract. The pollen is friable, and would not 

 of itself adhere to insects, but the moment the rostellum 

 is touched a drop of viscid fluid exudes on each side, 

 and when the insect retires it is sure to touch the 

 anther and carry off" the pollen. It is principally visited 

 by ichneumons, small beetles, and flies. Darwin several 

 times saw insects enter the flower and afterwards retire 

 with a pair of bright yellow poUinia sticking to their 

 foreheads. The number of insect visitors is so large, 

 having regard to the inconspicuousness of the flower, 



