DICHOGAMOUS PLANTS. 



[CHAP. 



do easily, though on the other hand, from the same 

 cause, it is impossible for them to return. Thus they 

 are imprisoned in the flower ; gradually, however, the 

 pistil passes maturity, and the stigma ceases to be capa- 

 ble of fertilisation, while the stamens ripen and shed 

 their pollen, by which the flies get thoroughly dusted. 

 Then the hairs of the tube shrivel up and release the 

 prisoners, which carry the pollen to another flower. 



Again, in the common Arum, we find 

 a somewhat similar mode of fertilisation. 

 The well-known green leaf, as shown in 

 the annexed diagrammatic figure (Fig. 

 29), encloses a central pillar which sup- 

 ports a number of stigmas (Fig. 29, st.) 

 near the base, and of anthers (a) some- 

 what higher. Now in this case nothing 

 would at first sight seem easier or more 

 natural than that the pollen from the 

 anthers should fall on, and fertilise, the 

 pistils. This, however, is not what occurs. 

 The stigmas mature before the anthers, 

 and by the time the pollen is shed, have 

 become incapable of fertilisation. It 

 FIG. 29. Diagram- is impossible, therefore, that the plant 

 should fertilise itself. Nor can the pol- 

 len be carried by wind. When it is 

 shed it drops to the bottom of the tube, 

 where it is so effectually sheltered that nothing short 

 of a hurricane could dislodge it ; and although Arum 

 is common enough, still the chances against any of 

 the pollen so dislodged being blown into the tube 

 of another plant would be immense. 



a, anthers ; 

 stigmas. 



st, 



