13 



tropic and bends away from the earth. In 

 this way these responses to the geotropic 

 stimulus counteract the influence of the 

 limiting factor. Roots behave similarly in 

 response to moisture content of the soil; 

 stems and leaves in response to light. 



In plants it is hard to draw a line be- 

 tween simple responses to stimuli and 

 morphogenic responses which involve per- 

 manent changes of form and structure. 

 The difference between sun leaves and 

 shade leaves is a familiar example of a 

 morphogenic response. The shape, size and 

 structure of the leaf here counteract the 

 limiting factor light. Again, plants which 

 are shaded by others so that they receive 

 insufficient light usually become etiolated, 

 that is, the stems and leaf -petioles in many 

 cases increase in length until some portion 

 of the plant is brought to a position where 

 it receives adequate illumination. Here 

 again the limiting factor is light, and the 

 result of etiolation is to overcome its effect. 

 Evolution is likewise an integrating 

 process. Its results are not accomplished 

 in the individual, but in the race, and are 

 called adaptations. Adaptations are means 

 of avoiding the effects of limiting factors. 

 Another means of integration is seen in 

 biotic succession. Here the integration ex- 

 tends over a considerable period of time 

 and its benefits do not accrue to the individ- 

 ual or the race, but to succeeding genera- 

 tions and different species. The integra- 

 tive effect in succession may be largely pro- 

 duced by the death and decay of an asso- 

 ciation resulting in the accumulation of 

 humus. Thus both xerophytic and hydro- 

 phytie plants prepare the way for a meso- 

 phytic flora. The limiting factor here is 



