The cnnse of plagiotropy in maritime shore plants 17 



Table V. 





0 mill. 



60 



120 



180 



Final 

 position 



Deviation 



1 



upper angle 



27, 30 



+ 11, ±0 



± 0, +30 



+ 6+0 



44, 60 



+ 17, + 30 



lower angle 



96, 67 



— 4, ±0 



+ 10, ± 0 



+ 2+0 



88, 67 



+ 8, + 0 



2 



upper angle 



31, 34 



+ 3, +4 



+ 13, + 7 



+ 18+5 



65, 50 



+ 34, +16 



lower angle 



83, 67 



±0,-2 



± 0, + 4 



+ 8+0 



80, 65 



+ 3, + 2 



The apices of the branches show the strongest upward movements (see also 

 Table HI); the basal parts follow at a slower rate. One of the apices on the first 

 plant made an upward movement of 11° after 60 minutes, while the basal part 

 oP the same branch only began to rise in the course of the second hour. This 

 part even showed a downward movement of 4° during the first 60 miu. We find the 

 same condition in the second plant. It is clear that the upward movement should 

 start in the zone which has the greatest rate of growth, if the geotropic sensibility 

 were of equal strength all along the branch. In as much as the rate of growth in 

 Afriplex is greater in the apical regions of the branches than in the basal parts 

 we should accordingly expect a shorter reaction time for the apex than for the 

 base where growth is slow. But this does not explain the dotvmvard movement in 

 the basal part of the branch during the first hour. Light, however, induces down- 

 ward movements of the branches, as will be further shown in the following tables. 

 Seeing that reaction takes place in the apex as soon as the plant is darkened 

 while the basal part continues to grow for some time in the direction determined 

 by the hght conditions previous to the darkening, it seems reasonable to assume 

 a transmission of the geotropic stimulus from the apical region where perception 

 first takes place down to the base which thus receives the stimulus later than the 

 apex. Consequently, the upper part bends first, and afterwards the bending gra- 

 dually extends down to the base. It should be noted that Atriplex prostratum Bouch 

 and the prostrate forms of A. latifolüim, A. patulum, and A. Babingtonii have the 

 nodes greatly swollen and distended as distinguished from the erect forms, and 

 this is particularly true of the points of insertion of the lateral branches on the 

 chief axis. The power of geotropic response gradually disappears in the lower 

 internodes in proportion as these cease growing, but is retained in the distended 

 nodes and enables the old branches, sometimes more than one metre in length, to 

 carry out geotropic curvatures even up to the fruiting stage. 



If now a plant whose above-ground parts have taken up the vertical position 

 in diffuse hght or in darkness is exposed to bright light or to direct sunlight, a 

 downward movement of the branches immediately results. Tliis downward move- 

 ment may be slow or rapid according to the hght intensity and the power of re- 

 action of the plant. — In order to obtain a relative measure of the intensity of 



3 



