FACTS 161 



considerable masses as with plant cells, a great number of 

 cell-tactisms taken together may result in giving an obvious 

 direction to the growth of a branch ; this is called tropism. 

 Examples are heliotropism, the turning toward the sun of 

 the sun-flower or heliotrope ; phototropism, as in a potato- 

 plant sprouting in a cellar and turning toward a hole through 

 which light passes ; geotropism, general in plants whose roots 

 turn toward the centre of the earth, while the stalk mounts 

 in an opposite direction. Tropism is a phenomenon of the 

 same order as tactism. One type of well-made experi- 

 ments in tactism will be enough for the reader to imagine 

 others for himself. 



From every point of view Pf effer's experiments are a model 

 of scientific exactness. Let us limit ourselves to the one 

 made on the chemico-tactic influence of malic acid on the 

 antherozoids of ferns . Fern antherozoids are small corpuscles 

 extremely mobile in water, where they are observed. Prob- 

 ably their mobility is due to a great number of different causes 

 and, by making one of these causes preponderant, we do 

 not destroy the effects of the others. The movements are 

 still very varied, but the particular development of their 

 composition of forces is due to the action of the factor which 

 has been introduced purposely. 



Pfeffer introduced a dosed solution of malic acid into a 

 small capillary tube, which he afterwards closed at one end. 

 The open end of this tube he plunged into an infusion in 

 which fern antherozoids were floating. It is clear that, 

 under such conditions, the malic acid will be diffused through 

 the infusion starting from the orifice of the capillary 

 tube. The diffusion will naturally take place in such a man- 

 ner that, at each successive instant, equal values of malic 

 acid will be distributed through the liquid of the infusion in 

 concentric spheres from the centre (Fig, 9, S p S 2 , S 3 ). 



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