62 



INFLUENCE OF CHEMICALS 



of the tubes in the two instances. Pollen tubes are apochemo- 

 tropic to the amount of oxygen in the air (20 per cent.) and direct 

 their tips away from atmospheres with this pressure of the gas. 

 Sealing the preparation with vaseline should exclude the gas and 

 allow the tubes to grow in all directions. 



97. Chemotropic Stimulation of Stigmas for Pollen Tubes. 

 Make a culture medium for pollen grains as above, using only 



Fig. 27. Chemotropic reactions of pollen tubes in penetrating the stomata of 

 leaves injected with sugar. Upper figure, hyphae of Phycomyces nitens penetrating 

 leaf injected with two per-cent. dextrin solution, 25 hours after beginning of experi- 

 ment. X 2 °' Lower figure, branching hyphae of Penicillium glaucum penetrating 

 stomata of leaf of Tradescantia 25 hours after beginning of experiment. X 7°- 

 After Miyoshi. 



so much sugar as may be sufficient for the particular species 

 under test (3 to 1 5 per cent.). Cut a small section from the surface 

 of the stigma of the same plant and place near one edge of the 

 culture fluid. Add a number of pollen grains, ring one with 

 vaseline and leave a second open. Note the directions ,of the 

 pollen tubes a day later. 1 



1 Lidtforss. Ueber den Chemotropismus der Pollen schlauche. 

 Gesell. 17 : 235. 1899. 



Ber. Deut. Bot. 



