22 TROPISMS 



organism as a whole from the individual reflexes, such 

 an attempt is in reality doomed to failure, since reactions 

 produced in an isolated element cannot be counted upon 

 to occur when the same element is part of the whole, on 

 account of the mutual inhibitions which the different 

 parts of the organism produce upon each other when in 

 organic connection 1 *; and it is, therefore, impossible to 

 express the conduct of a whole animal as the algebraic 

 sum of the reflexes of its isolated segments. 



E. P. Lyon 320 has shown that if the tail in a normal 

 shark be bent to one side (without changing the position 

 of the head) the eyes of the animal move as promptly 

 as compass needles in association with the bent tail 

 around the same axis in which the bending occurs, but in 

 an opposite sense. On the convex side of the animal, the 

 white of the eye is more visible in front, on the concave 

 side it is more visible behind ; hence the former has moved 

 backward, the latter forward. This was observed not 

 only in the normal fish but also when the optic and audi- 

 tory nerves were cut. The central nervous system acts 

 as one unit. R. Magnus 332 and his fellow-workers have 

 shown that an alteration in the position of the head of a 

 dog inevitably alters the tone of the muscles of the legs. 

 These and other associations and mutual inhibitions make 

 possible that simplification which allows us to treat the 



b When the stem of a plant (e.g., Bryophyllum) is cut into as many 

 pieces as there are nodes, each node will under the proper conditions give 

 rise to one or two shoots. If we leave them in connection, only the buds 

 at the apical end will grow out, the rest of the buds remaining dormant. 

 The whole stem acts as though it consisted of only the bud situated at the 

 apex. 



c The problem of coordination will form the subject of another volume in 

 this series by Professor A. R. Moore, and for this reason a fuller discussion 

 of work on coordination, such as that by Sherrington and by v. Uexkiill, 

 may be reserved for Professor Moore's volume. 



