476 MB. HAROLD WAGEE ON THE 



one side to the ofcher, across a drop o£ water a quarter of an incli 

 in diameter. I£ in their passage across, the sun becomes obscured 

 by clouds, or if a sheet of note-paper is placed between the sun 

 and the drop of water, they stop at once, and either remain 

 moving about in the middle of the drop or go back again to the 

 side nearest the source of light. 



If they are kept in bright sunlight for any length of time, they 

 €ome to rest and round themselves off ; and if they remain in a 

 good light for some days, the cells gradually become encysted. 

 The eifect of a strong light is always to produce encystment. 



In a light of moderate intensity, the cells may remain motile for 

 a very long time ; but they always become rounded off at night, 

 and may then lose their flagslla and undergo division into two. 

 Until they lose their flagella, however, they may be always 

 brought into the motile condition again by exposing them to 

 the light of a lamp or gas. This shows, as Klebs* has pointed 

 out, that they do not lose their sensitiveness even in the dark so 

 long as they can move. 



These experiments are sufficient to show that Euglena possesses 

 a very definite light-perception. Engelmann has also shown 

 that it is the colourless anterior end of the cell which is sensitive 

 to light, and it is only when this comes into contact with light 

 or shadow, that the cell reacts to the light by altering the 

 direction of its movements. If the shadow falls upon the 

 posterior chlorophyll-containing end of the cell, there is no 

 reaction. He has further shown that Euglena prefers the blue 

 portion of the spectrum, the following being the percentage dis- 

 tribution of a typical case : — 



Eed to Orange 1*4 per cent. 



Orange to Green 



G-reen 11-2 



Green to Blue 70-4 



Blue to Indigo 16"8 



Indigo to Violet 2'1 



It has also been shown that swarmspores are more sensitive 

 to the blue rays of the spectrum than to other parts. 



According to Franze (Z. c.) and "Wildeman t motile Euglence are 

 sensitive to heat, but they do not respond to it either so quickly 

 or so intensely as to light. 



* Loc. cit. p. 263. 



t " Sur le thermotaxisme des Euglenes," Bull. Soc. Micros. Belg. 1894. 



