HELIOTROPISM IN EUGLENA 419 



The best criterion that we have for determining that changes 

 of intensity constitute the stimulus is that the reaction shall be 

 produced when the change of intensity is rapid but shall not be 

 produced when the change is made gradually. This is the usual 

 criterion employed in electrical stmiulation. Its validity has 

 been pointed out for light stimulation by Loeb ('97, p. 439-441) 

 and it has been used by Jennings ('04, p. 52) and by Mast ('11, p. 

 94) for the shock-movements of Euglena. While this criterion 

 is perfectly satisfactory and conclusive wherever it is applicable 

 there are reactions to which it cannot be applied. Accordingly, 

 in these cases, other less satisfactory criteria have to be employed, 

 and we may now pass to an examination of some of these. 



Mast says ('11, p. 215) "In Euglena .... itwasdemon- 

 strated that the orienting stimulus is due to a change of light 

 intensity." But so far as I can see all the evidence adduced is: 

 (1) Orientation takes place by shock-movements for which the 

 stimulus is acknowledged to be change of intensity; (2) As the 

 Euglena revolves on its long axis some of its organs are subjected 

 to changes of illumination. The sudden swerving which is re- 

 sponsible for orientation bears a definite relation to these changes 

 and hence must be due to them (Mast '11, p. 104). The first 

 argument has already been disproved in the previous chapter. 

 The second is of no value in this connection because the changes 

 in illumination also subject the photosensitive organs to light of 

 different absolute intensities. It may well be that the continuous 

 action of strong light^^ causes the sudden swerving while neither 

 the action of the weaker light nor the change of intensity is 

 capable of producing the reaction. The light may exert its con- 

 tinuous action even when the intensity is not constant. The law 

 of Bunsen and Roscoe may still hold even when the light intensity 

 varies. 



present state of knowledge is contained in the paper of Blaauw to which Mast 

 refers in his book in another connection. The fact that Mast could make such a 

 statement as that quoted shows the need for the consideration of the criteria for 

 distinguishing these two reactions. 



1^ Lasting of course only for the fraction of a second during which the photosen- 

 sitive organ is turned towards the light. 



