MOLLUSKS, ARTHROPODS AND VERTEBRATES 229 



results and conclusions reached. Here we aim only to 

 bring together the more important factors involved in the 

 process of orientation in different organisms. 



(i) The plumules of Zea mays and probably of all the 

 other gramineae bend toward the more highly illuminated 

 side of the sensitive region regardless of the direction of the 

 rays. Our experimental results do not bear on the question 

 as to whether or not orientation is due to a modification of 

 circumnutating movements as maintained by Darwin. Nor 

 do they warrant a conclusion as to whether the stimulation 

 causing bending is due to a change of intensity on some 

 part of the sensitive region in accord with Darwin's sug- 

 gestion, or to constant intensity, all sides being continuously 

 stimulated in proportion to the intensity to which they are 

 exposed, in accord with the theories of De Candolle, Loeb, 

 Verworn and others. The bending may be due to a dif- 

 ferential response to a localized stimulation. 



(2) In the myxomycetes and rhizopods orientation is in 

 all probability due to a local response to a local stimula- 

 tion. Light retards the activity of the protoplasm and 

 thus prevents the formation of pseudopods on the more 

 highly illuminated side. It is impossible to say whether 

 light acts constantly as a directive stimulation, all parts of 

 the organism being continuously stimulated in proportion 

 to the absolute intensity to which they are exposed (Loeb, 

 Verworn, etc.), or whether it acts only through changes of 

 intensity, the prevention of formation of pseudopods on the 

 illuminated side being due to the increase in light intensity 

 on the protoplasm as the pseudopods are thrust out. 



(3) In Euglena, Stentor, Trachelomonas, Chlamydomo- 

 nas, Chlorogonium, Oedogonium swarm spores and prob- 

 ably in all other ciliates and flagellates which orient in 

 light, orientation is due to definite responses to changes of 

 light Intensity on the sensitive part of the organism. The 

 changes of intensity are ordinarily due to the movement of 

 shadows of one part of the body over another part. This 

 is caused largely by the rotation on the long axis. In 



