GENERAL 159 



concentration. Similar relationships may often be responsible for the 

 changed reaction produced by increasing intensity of stimulation. 



There is no reason, however, for assuming that all tropic or more 

 especially all plagiotropic reactions involve the antagonism of two or 

 more dissimilar stimulatory actions and responses. A single action may 

 induce or modify movement in organisms as well as in machines. Thus 

 the continued turning of a steam-cock (increasing stimulation) may induce 

 first a forward and then a backward movement of a locomotive. The 

 admission of steam into the cylinders can, however, produce no movement 

 if the wheels are fixed ; and in the same way an organ may be non-geotropic 

 or non-heliotropic either because the motor mechanism or the perceptive . 

 mechanism is undeveloped or out of gear, or because the connecting links 

 between the two are incomplete. 



Even in simple cases it is often difficult to determine whether a 

 particular plagiotropic position results from a tropic action alone or 

 involves other co-operating factors, and many instances of such conjoint 

 action are known. The parallelo-heliotropism or -the parallelo-geotropism 

 of an organ are easily determined separately, and hence it is possible to show 

 that the plagiotropic position assumed by certain organs under horizontal 

 illumination is the result of the co-operation of negative parallelo-geotropism, 

 and positive parallelo-heliotropism. In other cases the plagiotropism of 

 a shoot may be due to the interaction of its negative geotropism and 

 autogenic epinasty, the latter permanently preventing the assumption of 

 a parallelotropic position. When the stimulus of gravity is eliminated 

 on a klinostat, the epinastic curvature continues until the autogenic 

 campylotropism is fully satisfied. If gravity is once more allowed to 

 act the campylotropic curvature is decreased by the negatively geotropic 

 reaction, but is increased when the stem is inverted until the plagiotropic 

 position is once more assumed. Similar results may be obtained when 

 a growing branch is split longitudinally for a portion of its length, for 

 each of the outwardly curving halves shows an autogenic epinasty. If an 

 organ is placed so that the epinastic curvature takes place horizontally, the 

 geotropic reaction takes place at right angles to the curvature, so that an 

 obliquely ascending curve is performed. 



A plagiotropic position can equally well result from the co-operation of 

 autogenic epinasty with plagio-geotropism, as is actually the case in many 

 foliage-leaves. The pronounced backward curvature which these often 

 show on a klinostat demonstrates their autogenic campylotropism, and 

 also shows the part played by gravity in their plagiotropic orientation ; 

 for when the stimulus of gravity again acts the leaves raise themselves 

 into a horizontal position. If the leaf is pointed vertically upward it 

 descends into the plagiotropic position, which results from klino-geotropism 

 and epinasty, not from negative parallelo-geotropism and epinasty. 





