338 



EFFECT OF CHEMICAL AGENTS 



[Ch. XI 



were abundantly confirmed by MrrosHi ('94"), so we must 

 conclude that the pollen tube is negatively aerotropic to oxy- 

 gen. However, this negative aerotropism 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 in a 7% sugar solution and turn at the edge, u, 6, 

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

 Cephalanthera pallens, after 20 hours; «, 6, edge of caver-glass. u. 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. 



