338 



EFFECT OF CHEMICAL AGENTS 



[Cn. XI 



were abundantly confirmed by MIYOSHI ('94 a ), so we must 

 conclude that the pollen tube is negatively aerotropic to oxy- 

 gen. However, this negative aerotropisrn does not occur in all 

 pollen, for that of Orobus vernus and various other legumes, of 

 Primula acaulis, Viola odorata, V. hirta, etc., were indifferent. 



FIG. 92. Illustrates chemotropism of pollen-tubes, a. Negative chemotropism with 

 reference to the air (aerotropism) of pollen-tubes of Narcissus tazetta ; the tubes 

 are growing under a cover-glass iu a 1% sugar solution and turn at the edge, a, b, 

 from the air; magnified about 20. 6. Negative aerotropism of pollen-tubes of 

 Cephalanthera pallens, after 20 hours; a, b, edge of cover-glass. c. Stigma of 

 Narcissus tazetta in 7% sugar solution ; pollen-tubes grow towards the stigma ; 

 magnified about 10. (From MOLISCH, '93) . 



A second class of chemotropisms is seen in the turning of 

 pollen-tubes towards the stigma of a flower.* When pollen is 

 sown upon a plate of agar-agar or gelatine on which the upper 

 end of a ripe pistil has been placed, the tubes are sent out in 



* MOLISCH accounts for the failure of some of the earlier experiments with 

 pollen-tubes on the ground that certain pollen-tubes do not exhibit this class of 

 chemotropism. Among these are Viola odorata, V. hirta, Orobus vernus, etc. 

 species which are likewise not aerotropic. 



