GROWTH AND WORK OF PLANTS 



disk. When the insect alights on the labellum and touches a 

 sensitive structure, the pollinium is set free, the tension of the 

 curved stalk causes it to fly out like a spring, and the viscid 

 disk attaches itself to the head or to the proboscis of the 

 insect. In this position it stands pointing upward, holding the 

 pollinium too high to touch the stigmatic surface of that flower. 

 In the course of half a minute or a minute, the stalk curves down- 

 ward in such a position that the pollinium will strike the stig- 

 matic surface. But by this time the insect has gone to another 

 flow r er and cross-pollination is effected.* In members of the pea 

 family (plants like Cytisus, Spartium, etc., grown in greenhouses 

 are excellent to experiment with) the pistils and stamens are held 

 in the keel. When an insect alights on the keel the style suddenly 

 flies upward and the brush of hairs throws out the pollen on to 

 the body of the insect. When it visits another flower in which 

 the stigma shows above the keel and is receptive, some of the pollen 

 is brushed on to the stigma. In the canna flower the pollen is 

 shed from the anther (while the flower is still closed) and glued 

 on to one side of the broad style right by the side of the stigmatic 

 surface but not on it. As the flower partly opens, bumblebees 

 alight on the lower petal (labellum), which suddenly curves down- 

 ward, taking the bee far below the stigmatic surface. As the bee 

 enters the flower it brushes against the pollen mass and removes 

 some of it, but as it was immediately lowered it could not rub 

 this pollen onto the stigma while it was taking the nectar. But 

 as it visits the next flower, when it first enters some of the pollen 

 from the previous flow r er is brushed off on the stigma. Interesting 

 experiments can be made on some of the orchids grow r n in green- 

 houses with a lead pencil or other slender-pointed instrument to 

 imitate the movement of the proboscis of an insect. Some of the 

 native orchids can be used to demonstrate the methods of cross- 

 pollination. 



296. Flowers constructed to lead the insect in at one 

 point and out at another. Some of our native species illustrate 



* Darwin's work "On the Fertilization of Orchids by Insects" should be 

 consulted. Full descriptions and illustrations are given. 



