XI] 



ITS GENERAL PROPERTIES 



497 



It is, as we have so often seen, an essential part of our whole 

 problem, to try to understand what distribution of forces is capable 

 of producing this or that organic form,^to give, in short, a 

 dynamical expression to our descriptive morphology. Now the 

 general distribution of forces which lead to the formation of a 

 spiral (whether logarithmic or other) is very easily understood; 

 and need not carry us beyond the use of very elementary mathe- 

 matics. 



If we imagine growth to act in a perpendicular direction, as for 

 example the upward force of growth in a growing stem {OA), then. 



Fig. 239. 



in the absence of other forces, elongation will as a matter of course 

 proceed in an unchanging direction, that is to say the stem will 

 grow straight upwards. Suppose now that there be some constant 

 external force, such as the wind, impinging on the growing stem; 

 and suppose (for simplicity's sake) that this external force be in a 

 constant direction (^5) perpendicular to theintrinsic force of growth. 

 The direction of actual growth will be in the line of the resultant 

 of the two forces : and, since the external force is (by hypothesis) 

 constant in direction, while the internal force tends always to act in 

 the line of actual growth, it is obvious that our growing organism 

 will tend to be bent into a curve, to which, for the time being, 



T. G. 32 



