EYE- SPOT AND FLAGELLTJM IN EUGLENA VIRIDIS. 477 



To determine tlie effect of heat, Franze placed Euglents in a 

 thin glass tube which was closed at both ends, and kept warm 

 at one end by allowing a stream of hot water to flow over it. 

 The tube was kept in the darkl At a temperature of 55° C, the 

 Euglencd became immobile from the effects of heat, but at a 

 lower temperature, 30°-40° C, they mostly approached the source 

 of heat. 



Wildeman obtained somewhat similar results. He placed 

 Muglenw in a tube with wet sand, in order to avoid convection- 

 currents. It was then placed in the dark in a horizontal 

 position, and warmed at one end, and he found that the Euglence 

 accumulated at the warmer end of the tube at a temperature of 

 30° C. On exposing tubes to light as well as heat, he found a 

 considerable modification in the effects produced. If they were 

 placed at right angles to the rays of light, the JEuglence avoided 

 the warm end of the tube. If they were placed in the same 

 direction as the rays of light they moved towards the light, even 

 when the opposite end of the tube was heated. 



My own experiments confirm in a general way the results 

 obtained by these observers ; but the Euglence are so slightly 

 sensitive to heat as compared with light, that the subject requires 

 further investigation before any very definite conclusions can be 

 arrived at. 



The Function of the Eye-spot. 



It is commonly stated that the eye-spot is a light-perceiving 

 organ. This does not imply that it possesses an actual visual 

 function, but simply that it is connected in some way with those 

 changes in the direction of the movements of the cell which are 

 due to light. There is no direct proof of this, but we have a 

 sufficient amount of indirect evidence to show that the statement 

 is probably a correct one. 



Ehrenberg regarded the pigment-spot in Euglena as a light- 

 perceiving organ on account of its general resemblance to the 

 eyes of Rotifers and Cyclops; and it has since been shown 

 that it resembles them in some respects both in structure and in 

 its behaviour towards solvents and other reagents, such as iodine 

 and sulphuric acid *. 



Purther, in all those chlorophyll-containing unicellular or- 

 ganisms which are very sensitive to light and capable, by means 

 * Klebs, loc. cit. 



