HOW PLANTS LIVE AND WORK. 335 



head as it stands on the lower Up of the flower. When it pushes 

 forward its head to reach the honey, it comes in contact with the 

 lower pollen-boxes, and the cross-pieces swing round on their 

 hinges, bringing the upper pollen-boxes down with a smack on the 

 bee's back, and sprinkling it liberally with pollen dust. Having 

 shed their pollen, the stamens shrivel up, and the pistil comes to 

 maturity. As it ripens, the stigma arches over, so as to scrape 

 along the back of any bee visiting the flower for the honey, and 

 thus to wipe off the pollen which has been brought from a 

 younger flower. 



The Orchids form another group which has invented a novel 

 and ingenious method of securing cross-fertilisation. The pollen 

 is present in these flowers when ripe as two club-shaped masses. 

 The " handles " of the clubs are connected with a slightly com- 

 plicated, but exceedingly sticky, arrangement, by which a moth 

 coming to suck the juices of the flower is practically compelled 

 to carry off the pollen masses on its proboscis. When the moth 

 leaves the orchid, these pollen masses have an upward direction, 

 but they soon begin to point forward, because the membrane to 

 which they are attached contracts in a peculiar way. They now 

 project in such a manner that when the moth visits another flower 

 the masses of pollen come in contact with its stigmatic surfaces. 

 These surfaces are also gummy, and tear off some of the packets 

 of pollen of which the " clubs " are composed. A single pair of 

 pollen masses is thus sufficient to fertilise several Orchids. We 

 may readily imitate the whole process by pushing a fine straw, or 

 the point of a pencil, into the flower. The pollen masses stick to 

 the straw, and are removed when this is withdrawn. 



It should now be clear that the flowers of such plants as are 

 fertilised by insects are modified in accordance with the habits and 

 structure of their guests. For example, flowers which are fertilised 

 by night-flying moths have no honey-guides, and they generally 

 keep closed, or " sleep," during the day. 



The insects, in their turn, have become modified to harmonise 

 with the peculiarities of the flowers they most affect. This change 

 of structure is, as one would suppose, most marked in the mouth- 

 parts and legs of bees and butterflies. Those bees which visit 

 flowers whose honey is at or near the surface have short lower lips, 



