352 THE BIOLOGY OF FLOWERING PLANTS 



of cases to prove that there is no general correspondence. 

 His criterion of damage is, however, the power of ger- 

 minating in water and not the power of germinating in a 

 suitable solution after exposure to water, and this vitiates 

 his results. In the same work he finds no relation between 

 floral movement and sensitiveness. 



There is evidently a wide field for research here, and it is 

 likely that results of more value will be obtained by intensive 

 study of a few carefully chosen types than by cursory 

 examination of such large numbers as are dealt with by these 

 two investigators. 



§ 10. Pollination and Fertilisation 



The pollen grain of the angiosperms is received on the 

 receptive surface of the stigma which is often roughened by 

 papillae and so tends to hold the grains. It is often viscid, 

 too, a condition which both holds the grains and promotes 

 germination. This occurs without delay, and the pollen 

 tube grows into the tissue of the stigma, down through the 

 style, and so into the ovarial cavity. The tube may actually 

 pass through the cells, dissolving the cell walls by enzymatic 

 secretions, or it may grow through the spaces of a special 

 loose tissue, or through a well-defined canal. The direction 

 of its growth seems to be determined partly by negative 

 aerotropism, and partly by positive chemotropism. Pollen 

 grains germinating on gelatine grow into the medium away 

 from the oxygen of the air. It has been shown by Molisch 

 (1893), Miyoshi (1894), and Lidfors (1899, 1909), that the 

 tubes grow towards pieces of stigma or ovules. The active 

 substances are usually sugars. In Narcissus Tazetta and 

 many other plants, Lidfors showed that a protein was the 

 directing chemical. In the ovarial cavity the tube grows 

 along the placenta and enters the ovule by the micropyle ; 

 it pierces the nucellar tissue and the wall of the embryo 

 sa3 ; its own wall is broken down and the conditions for 

 fertilisation are realised. Incidentally we may note that 



