■ 192 Smith : Peculiar Case of contact Irritability 



The influence of the two stimuli, thermotropism and hydrotropism, 

 must therefore be excepted. 



Two pots were put into the same box with an electric fan 

 keeping the air constantly in motion. The pot on the clinostat 

 showed parallel adjustment very well (Fig. 10), also the stationary 

 one (Fig. 1 1). The stillness of the air, therefore, makes no 

 difference. 



Chemotropism and hydrotropism, exerted by the leaves of 



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neighboring seedlings, might furnish a stimulus but were not experi- 

 mented with, because the stimulus to be described seemed sufficient 

 to account for the phenomenon. 



Of the mechanical forces, acting as irritable stimuli, pressure, 

 strain and injury were not present, but certain observations made 

 contact seem a possible factor and experiments were devised to 

 test this. 



Sets of beans were carefully matched and their positions plot- 

 ted every other day. The plane of petioles and leaves was con- 

 stantly changing but the final position was in a number of cases 

 the position of the bean which first started from the cotyledon or 

 at least the resultant position was the same for leaves which had 

 been in contact. There was no chance for any contact in beans 

 planted in 1 2-inch pots and the final positions showed no parallel 

 adjustment. 



Beans which were fully grown were placed so that the tip of 

 the leaf of one seedling touched the leaf of another seedling. 

 The diagrams 12, 13, 14 and 15 show the change of position. 



The tendency as here shown is to take a parallel position. 

 The movement is best seen when seedlings are not quite full crown. 

 If contact can then change the position of the leaves, it must be 

 possible to change the position at will. Paper leaves were fastened 

 to standards and allowed to just barely touch the leaves of a seed- 

 ling (Fig. 16, a). An angle of 25 was reached in all cases (Fig. 

 ]6 t 6). (Beans, kept in absolute darkness and watered by an 

 outside tube did not unfold the plumule, but showed a movement 

 of a quarter of an inch from the paper leaves.) Circumnutation 

 alone, of course, could not bring about this definite fixed position. 



Contact seems to work when light is present but is generally 

 marked by the more noticeable light adjustment. One pot of 



