1 8 LIGHT AND THE BEHAVIOR OF ORGANISMS 



lite differ entielle.'' The valuable experiments of Engelmann 

 on the behavior of unicellular organisms in microspectra 

 will be considered later (see Part IV). 



Several very important contributions to the knowledge 

 of the reaction of plants, both in theory and in fact, were 

 made by Charles Darwin and his son Francis, in their 

 excellent work on " The Power of Movement in Plants " 

 (1880). (i) They made detailed observations on the move- 

 ment of different parts of plants in the absence of definite 

 external stimulations, and found that practically all parts 

 of plants — stems, leaves, roots, flowers, etc. — are constantly 

 performing circumnutation movements. From this they 

 concluded that tropic curvatures are brought about by 

 modification of movements already present, i.e., that tropic 

 stimuli are not the cause of movement but the cause of 

 modification of movement. (2) They studied the reaction 

 to light of plumules with the tips covered with small opaque 

 caps; of radicles with the tips cauterized by the application 

 of silver nitrate; and the reactions to gravity of radicles 

 with the tips removed, and found that these structures 

 responded normally after the tips were covered, removed 

 or injured, provided that they had been previously stimu- 

 lated, but that they did not respond if they were not stimu- 

 lated until after the operation. From these results they 

 concluded that plant-organs frequently have a sensitive 

 part separated by some distance from a reacting part which 

 is not sensitive, and that impulses originating in the former 

 are transmitted to the latter. (3) They studied the reac- 

 tions to light of certain plumules with one side covered 

 with an opaque substance, and of others not covered but 

 exposed at intervals, and concluded that the reactions are 

 due to difference in intensity on opposite sides but that the 

 principal factor in producing stimulation is a change of 

 intensity rather than absolute difference of intensity. 



These conclusions are of such fundamental importance 

 that it seems advisable to insert the following quotations 

 from the authors' work cited above, (p. 485): "All ob- 



