The cause of plagiotropy in maritime shore plants 23 



From the facts recorded in the above tables it is evident that Prunus and 

 Cotoneaster carry out movements in response to changes in hght intensity; in ob- 

 scnre hght or in dari^ness towards a vertical position, in brigl)ter light towards a 

 horizontal position. Although strong winds may be responsible for some cases of 

 espalier-shaped postrate growth of woody plants along exposed coasts, the effects of 

 insolation on the orientation of stems should not be overlooked. 



Table XIV. 







Original 

 position 



Final position 



Deviation 



1 



Dark room 



(55 



57 



+ 8 



2 





74 



68 



+ 6 



3 



Diffuse light 



56 



54 



+ 



4 



s » 



83 



70 



+ 13 



5 



Direct sunlight 



8 



16 



— 8 



6 





68 



75 



— 7 



7 



i » 



54 



62 



— 8 



8 





70 



74 



— 4 



It is superfluous to give additional data to illustrate the significance of light- 

 intensity in regulating the position of the above-ground parts of the plants in ques- 

 tion. By varying the Hght intensity the horizontal and the vertical positions may 

 be produced at will. As to the question in what manner the movements are released 

 by light and darkness, clinostatic experiments must decide this. 



In these experiments only cultures of Atriplex prostratum Bough, were used. 

 The cultures were rotated on the horizontal clinostat axis in diffuse hght for three 

 weeks in order to neutrahze the effects of the so called epinastic curvatures. They 

 were then put in the dark room and ciinostated on the horizontal axis. No curva- 

 ture resulted in the plants treated in this way. The branches continued to grow in 

 the original direction in consequence of the autotropism of the plant, without show- 

 ing any sign of upward movements. The same rigidity was shown by cultures 

 exposed to intensive sunlight. However, when uncoupled from the clinostat and 

 placed in normal position downward movements immediately followed. 



It may be added that the Atriplex-hvSinch is radial anatomically. In obscure 

 light or in darkness it is, furthermore, radial physiologically. When strongly illu- 

 minated a physiological dorsiventrality is shown by the plagiotropic branches, since 

 such a culture placed in inverse position recovers the horizontal position by acce- 

 lerated growth of the morphological upper side. The original (morphological) upper 



