Smith : Peculiar Case of contact Irritability 193 



beans, planted in a circle, was kept revolving upon a clinostat in 

 light. Fig. 17 shows the final position. The parallel adjustment 

 was not so plain at first, but became more definitely marked with 

 the growth of the seedlings. 



One seedling of this set was kept, when the others were de- 

 stroyed and was irritated by two leaves of thin mica, fastened to a 

 stick placed in the pot. The whole thing was then revolved upon 

 a clinostat. By this means the direct influence of heliotropism 

 was neutralized and all shading from leaves was prevented. With 

 each of three trials the seedling cotyledons moved one half inch 

 from the artificial leaves. 



Under ordinary circumstances of growth the plants experi- 

 mented upon try to expose as much surface to the light as 

 possible for photosynthesis. Heliotropism is the stimulus for this 

 exposure, but, when light is withdrawn or, perhaps, during the 

 night, when growth is active, the plants must make use of some 

 other stimulus to determine other positions. Contact is always 

 present and is apparently the stimulus used by the plants. It is 

 probable that this influence is a secondary one, acting in light as 

 well as in darkness. The cases where currents of air were allowed 

 to act upon the plants showed good adjustment, for by means of 

 them the number of contacts of the leaves was increased. 



Description of Figure** 



Fig. I. Photograph showing parallel adjustment of petioles in bean seedlings 

 grown in the dark. 



Figs. 2, 3, 4. Diagrams of positions of seeds as planted in Exp. I. and the result- 

 ant positions of petioles. 



Figs. 5, 6. Diagrams illustrating planting of seeds in Exp. II. and resultant \ vi- 

 rions of petioles. 



Figs. 7, 8. Diagrams illustrating 1st part of Exp. III. 



Fig. 9. Diagram illustrating 2d part of Exp. III. 



Fig. 10. Diagram of resultant positions of petioles in seedlings, grown in dark 

 upon clinostat, acted upon by currents of air. 



FIG. II. Diagram of seedlings, grown in the dark and kept stationary, acted 



upon by currents of air. 



Figs. 12, 13, 14, 15. Diagrams of 4 examples of seedlings, whose leaves were 



placed in contact and the resultant changes in their positions. 



Fig. 16. Diagram of seedlings, whose leaves were irritated by paper leaves and 



the consequent changes in their positions. 



Fig. 17. Diagram of resultant position of petioles in seedlings, grown in light 



upon clinostat. 



