70 LIGHT AND THE BEHAVIOR OF ORGANISMS 



have been impossible to detect anything but rather de- 

 cided effects. It is hoped that these experiments may be 

 extended. 



e. Discussion. — The conclusion arrived at above that 

 orientation is regulated by the difference in light intensity 

 on opposite sides of the plumules is in direct opposition to 

 Sachs' theory (see p. 13) of orientation. It opposes that 

 of Loeb in so far as he attaches importance to the idea 

 that symmetrically situated points on the surface must be 

 struck by light at the same angle when the organism is 

 oriented (see p. 28). It neither confirms nor contradicts 

 Loeb's and Verworn's idea (see pp. 29, 38) as to the direct 

 effect of the external agent on the motory tissue. Nor does 

 it bear on the question proposed by Darwin (p. 18) that 

 orientation is due exclusively to modification of circum- 

 nutations. It is entirely possible that the lateral illumi- 

 nation causes an increase as well as a change in the direction 

 of the movement. 



Superficially the evidence seems to indicate clearly that 

 orientation is direct, that there is nothing corresponding 

 to selection of random movements (see p. 50). However, 

 it is impossible to say in how far even very slight circum- 

 nutating changes in position may affect diversity of light 

 intensity within the individual cells in the sensitive zone, 

 and in how far such changes in position may be interpreted 

 as trial movements. Owing to the possibility of such 

 variations in illumination within the cells, due to very 

 slight changes in the position of the plumule, it is also 

 impossible to decide whether the stimuli which cause 

 orientation are due to constant intensity or to change of 

 Intensity. 



These experiments have no bearing on the question as 

 to how curvature resulting in orientation in the plumules 

 is produced. The experiments of Darwin and others, how- 

 ever, showing that there Is a distinct sensory and motory 

 zone in these structures, demonstrate clearly that it is not 

 due to the direct effect of the illumination on the tissues 



