THE LEVELS OF BEHAVIOUR 141 



51. ON TROPISMS 



A tropism is a growth movement in response to a vector 

 stimulus (which is a stimulus that has direction as well as magni- 

 tude). Almost the only good examples of tropisms are the 

 growth movements of plants in response to the light that falls 

 on them, or in response to gravity. The green part of a seedling 

 grows vertically upwards in the direction in which the light falls 

 and the rootlet grows vertically downwards in the direction in 

 which the earth's gravitative force is exerted. The upw^ard 

 growth of the green shoot is called phototropism and the down- 

 ward growth of the rootlet is called geotropism. Again, the 

 leaves of a plant placed near to a window tend to turn so that 

 they receive the most favourable (or " optimum ") intensity of 

 light, that is, they turn so that the light may fall normally to 

 their green surfaces. These movements and others of the same 

 type are effected either by the cell-divisions of the growing 

 tissues occurring in planes such that the shoots, or rootlets, or 

 leaf-stems take up certain attitudes, or they are effected by 

 variations in the turgidities of the tissue-cells. 



Tropisms have " sign." The phototropism of the growing 

 plant is said to be positive when the growth occurs tow^ards the 

 source of light, negative when the growing plant turns away 

 from the source of light. The sign of the tropism is said to be 

 reversed when a change in the tensor, or magnitude, of the 

 stimulus (the direction of the latter remaining the same) 

 causes the direction of growth to be reversed. There is little 

 precision in these latter ideas, for good examples of tropisms 

 are few. 



There is purpose, in the sense used, in a tropism, for the 

 growth movements are such as to promote assimilation. With 

 variations in the direction of the incident light the rate of COo- 

 assimilation would sometimes fall off, but given such a change 

 of direction of growth as will maintain this rate constant, or as 

 nearly so as possible, then the tendency of the tropism is to 

 maintain normality. But there is not behaviour in the precise 

 sense which we have adopted, for the plant itself as a whole does 

 not move. There is simply change in the direction of growth 

 of some of its parts relatively to the growth of the other 

 parts. 



