THE WELCOME OF THE FLOWERS 



421 



hang together in strings. The same substance appears to be the cementing 

 vehicle in the majority of Orchids, where every pollen-sac (corresponding 

 to an anther-lobe in ordinary flowers) contains an agglutinated club-shaped 

 pollen-mass, known as a poUinium. The two pollinia commonly slope 

 clown to a little viscid gland or knob the rostellum. which readily adheres 

 to any object coming in contact with it, and which serves to mark the 

 frontier between the solitary stamen * and the stigmatic surface below it. 



In the Common Twayblade (Listera ovata, fig. 523) this organ is flat and 

 scale-like, and arches over the stigmatic surface. " Sprengel has correctly 

 described," says Hermann 

 Miiller, "how small insects 

 regularly alight on the 

 lower end of the labellum 

 (a), and slowly creep up- 

 wards, licking the honey in 

 the groove(%); when they 

 have finished and raise 

 their heads, they come in 

 contact without fail with 

 the slightly prominent 

 edge of the rostellum (r). 

 On the slightest touch, 

 this exudes a small white 

 drop of fluid, whichreaches 

 the apex of thepollinia(p), 

 and hardening instantly, 

 cements them to the ob- 

 ject whose touch caused 

 the exudation ; and so 

 in every flower which 

 has not previously been 

 visited, the insect visitor 

 receives a new pair of 

 pollinia. The insect flies 

 away startled, and soon 

 afterwards alights on 

 the labellum of another 

 flower, usually on another 

 plant." 



* Most species of Orchideae 

 are monandrous i.e. have only 

 one stamen in each flower ; but 

 a few species are diandrous. 



Photo by] 



[E. Step. 



FIG. 524. BROAD-LEAVED HELLEBOKINE 

 (Epipactis latifolia). 



A portion of the long raceme of this native Orchid. In this photograph the 

 parts of the flower may be seen clearly. It is visited by wasps principally. 



