CHEMOTAXIS OF ANTHEROZOIDS OF FERNS 59 



to some of these the organism exhibits only negative action and 

 moves out of the sphere of their influence as a reaction. Again 

 the organism may not be repulsed by the highest concentration 

 of the substance even when poisonous. 



The threshold of stimulation lies at a very low degree of con- 

 centration. After the organism is under the influence of any 

 chemical agent however, the additional intensity necessary to 

 constitute a stimulus will increase with the concentration already 

 existing, in accordance with Weber's law. Thus if an organism 

 is in distilled water it will react to a much smaller intensity of 

 action of sugar than if it were in a one-per-cent. solution of this 

 substance. 



94. Chemotaxis of Antherozoids of Ferns. Secure numbers of 

 prothallia of Adiantum, or some species which produces great 

 numbers of antheridia in comparison with the archegonia. A 

 crop of prothallia may be provided by sowing spores in a moist 

 chamber in a greenhouse two months before needed. Examine 

 from time to time and when the antheridia seem about mature 

 take a few of the prothallia from the soil and wash clean. Place 

 on a glass slip and cover with a small circle of glass, which 

 should be supported at the edges by small bits of glass or hairs. 

 Draw rain water through the preparation several times in order 

 to wash free from organic acids. Malic acid is present in the 

 cells, and if any have been ruptured the free acid would interfere 

 with the success of the experiment. Draw out some fine capil- 

 lary tubes until they are not more than .2 mm. in diameter. Cut 

 in sections about 8 mm. in length and close one end by fusion. 

 Make a one-tenth per-cent. solution of malic acid or sodium 

 malate in a small dish and lay the tubes in it. Set under the re- 

 ceiver of an air pump and exhaust. The expansion of the air 

 contained in the capillary tubes will allow some of it to escape 

 and when the dish is removed from the receiver, some of the so- 

 lution will run in to take its place. Select one of the capillary- 

 tubes and rinse it lightly in water for a moment, then thrust the 

 open end under the edge of the cover-glass into the water. The 



