Mast, Orientation in Euglena with some Remarks on Tropisms. 643 



of Bancroft to the contrary, concerning Loeb, notwithstanding. 

 Bancroft says (p. 387), "Loeb certainly never thought . . . that 

 'orientation is produced by the direct action of the stimulating 

 agent on the motor organs of that side of the body on which it 

 impinges' (1906, p. 266), . . . and has never written anything of 

 the kind so far as I have been able an discover"; and yet no 

 clearer or more direct statement of the theory has ever been made 

 than will be found in the quotation given above. In this case 

 Loeb does refer to orientation of plants but I assume he would 

 apply the same to animals, for he repeatedly says the process of 

 orientation in plants and animals is identical. 



Certain statements made by Torrey also indicate adherence 

 to this theory. He says, referring to Euglena (1907, p. 319), ,,In 

 heliotropism . . . the oriented organism is in a condition of phy- 

 siological stimulation, and . . . the response to stimulation is 

 local". Judging from this statement I concluded that he was 

 an advocate of the local-action theory, and made a statement 

 to that effect; but in a recent paper (1913, p. 874) Torrey 

 by way of criticism maintains that he never believed in this 

 theory. 



(2) The reflex-action theory is similar to the local-action theory. 

 The essential difference lies in the fact that it teaches that the 

 motor elements are stimulated indirectly through a reflex arc, not 

 directly as maintained in the local-action theory. This is the theory 

 most frequently advocated by Loeb in recent years. 



He says (1912, p. 38), "Two factors govern the progressive 

 movements of animals [in light] . . .; one is the symmetrical struc- 

 ture of the animal, and the second is the photochemical action of 

 light". In the words of Loeb (1906, p. 13f>), the orienting reac- 

 tion is, according to this theory, "a function of the constant inten- 

 sity". Formerly I assumed that Loeb meant by the phrase 

 "function of the constant intensity" that the stimulating agent ne- 

 cessarily must act continuously, i. e. without intermission. But 

 now I find (1910, p. 465) that he holds that this theory, in so far 

 as the nature of the stimulating agent is concerned, covers all 

 orienting reactions which are in accord wit the Bunsen-Roscoe 



to those forces which have a tendency to elongate or stretch the plant, e. g., hydro- 

 static pressure inside the cells, or imbibition of certain tissue elements. The otucome 

 will be that one side of the stem will be stretched more than the opposite side, 

 and this will bring about a curvature of the stem. Where the latter is soft at the 

 tip, the bending will occur only, or chiefly, in that region; and as the degree 

 of softness decreases rapidly from the tip downward, the result will be that the 

 tip will bend toward the source of light. This result may possibly be aided by a 

 greater photosensitiveness of the extreme tip of the stem, although I am not aware 

 that this is an established fact". 



