TACTIC RESPONSE TO TROPIC STIMULI 311 



of the body, the resultant shock-movement will naturally take place in the 

 new direction. Dorsiventral organisms like Paramoecium l assume definite 

 positions as the result of every shock-movement, but whether this also 

 applies to vegetable organisms is uncertain. 



Since we are dealing with two distinct forms of irritability, one agency 

 may induce a tactic and another a phobic movement, while in some cases 

 the same stimulus may excite both forms of response. Many Infusoria are 

 galvanotactic, but chemophobic and osmophobic, while certain Volvocineae 

 are phototactic and also osmophobic 2 . According to Carrey 3 Chilomonas 

 is chemophobic to the more active inorganic acids, and chemotactic to the 

 feebler organic acids. It is in fact possible that in many cases the chemo- 

 tactic attraction by weak solutions becomes a chemophobic repulsion when 

 they are more concentrated. 



In spite of the generally useful adaptive character of these responses, 

 it is not surprising that in many cases a galvanotactic irritability should be 

 shown, although it cannot have any practical importance. Similarly, 

 although many organisms avoid injurious concentrations, others swim into 

 these or even into poisonous solutions where they are killed. The best 

 chemotactic agency can only attract or repel across relatively small 

 distances, although light and gravity are more extended in their action. 

 For biological purposes of attraction tactic stimulation is in general more 

 advantageous, for the spermatozoids of Ferns, for instance, could hardly be 

 drawn with certainty in any other way to the ovum. Phobic stimulation 

 is, however, ample to attract and retain bacteria to special loci, or to 

 prevent their penetration into injurious media. 



Various orientations within the cell probably result from unilateral 

 stimulation, but hitherto only the phototactic movements of chloroplastids 

 and the traumatropic movements of the nucleus are known with certainty. 

 The protoplasmic aggregation which results from various stimuli may be 

 due either to a primary or secondary reaction a distinction difficult to 

 determine under the complicated relationships prevailing within the cell. 

 The slow progress of the internal movements afford, however, strong 

 evidence that they are not the result of shock-stimulation. In general, 

 the power of movement is antecedent to tropic stimulation, and its rapidity 

 is not perceptibly modified by the latter, although many instances may 

 ultimately be found to exist in which a latent power of movement is first 

 awakened by the tropic stimulus. Nageli 4 found, however, that the photo- 

 tactic stimulation of Algal zoospores, and Pfeffer 5 that the chemotactic 



1 Jennings, Am. Journ. of Physiol., 1899, Vol. II. 2 Rothert, 1. c., p. 396. 



3 Carrey, The effects of ions upon the aggregation of flagellated Infusoria, 1900. 

 * Nageli, Beitrage z. wiss. Bot., 1860, Heft ii, p. 102 ; Strasburger, Wirkung d. Lichts u. d. 

 Warme auf Schwarmsporen, 1878, p. 27. 



3 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 375. 



