Summary. 



1. A number of plants are found to exhibit a prostrate form of growtli on sea- 



shores and other exposed localities. It is not only the lateral shoots that lie 

 prostrate but the main axis itself bends down, lying prone on the soil. Caes- 

 pitose forms and rosette-plants have hkewise their basal leaves horizontally 

 expanded and firmly pressed to the ground when growing in such localities. 



2. The factor responsible for the prostrate growth has been found to be intensive 



light. Plagiotropy is most readily produced by exposing orthotropic cultures 

 to bright light. A further analysis has shown that the downward movements 

 induced by bright light are geotropic in nature. 



3. The term photocliny has been accepted for this particular kind of movement, 



released by gravity in response to variations in light intensity, as distinguished 

 from heliotropic movements. It is believed that this determining effect of light- 

 conditions on the nature of the geotropic response, found to be true of the 

 plants investigated, is a rule of universal application. 



4. Besides plagiotropic forms brought forth by the action of bright light there are 



also found, sometimes within the same » systematic species », hereditary plagio- 

 tropic forms. Such a parallelism between »climatic modifications» and »climatic 

 variations» has been found in the genus Atriplex. 



5. Photocliny (and psychrocliny) will probably in most cases be found to account 



for the prostrate growth of maritime and alpine plants. The existence of 

 hereditary prostrate races sliould not be overlooked. 



