Br acker, — Observations on Euglena deses. ioi 
the day appear to make no difference to the number of Euglenae on the 
surface, as in bright sunlight or good daylight without sun the greenness of 
the mud remains the same. If, however, the day is extremely dull and the 
intensity of the light does not reach a certain point required by the Euglenae, 
only a very few organisms appear, and from December 13 to December 15, 
when a very heavy fog enveloped the city, no individuals could be seen on 
any of the mud under observation. The Euglenae disappear into the mud at 
nightfall, but the same phenomenon can be brought about by placing them 
at any time in the dark. It was found that the time taken for complete 
disappearance varied with the temperature, as will be shown later, but 
averaged about thirty minutes. On replacing in the light the Euglenae took 
about the same time to rise to the surface again. 
If kept in the dark for one or two days the Euglenae do not appear on 
the surface until replaced in daylight. If kept for longer periods a few may 
be visible on the surface of the mud, and eventually most of them return to 
the surface, probably to obtain more oxygen. The chief difference in the 
appearance of these Euglenae kept in the dark is the disappearance of the 
paramylon grains which are used as reserve food, and when all has been 
used up the organism dies. On being replaced in light, however, before all 
the paramylon has disappeared, the Euglena is able to manufacture fresh 
grains in a very short period of time. 
Another experiment was performed in which the Euglenae were placed 
in dishes and illuminated only from one side in order to see if they would 
move towards the light. 
Four dishes were used which contained respectively : 
(1) A quantity of stiff mud containing Euglenae. 
(2) A quantity of wet mud containing Euglenae. 
(3) A quantity of mud with Euglenae, over which a layer of water 
half an inch deep was placed. 
(4) A number of Euglenae in water only. 
In (1) and (2) the Euglenae gradually moved towards the source of 
light, and about the end of five days were all massed at the lighter end of 
the dish, many heaped upon the glass. 
In (3) the Euglenae moved somewhat towards the source of light, but 
not so pronouncedly as in (1) and (2). 
In (4) the organisms kept their original position, and at the end of the 
day appeared completely dead, owing to a lack of oxygen and food material, 
as they are not able to swim in water. 
In the diagram given below (Fig. 6) the positions of the Euglenae at 
the beginning and end of the experiment are shown. 
From these experiments it may be concluded that Euglena deses is 
sensitive to light and moves towards the source of light, and is not repulsed 
by bright sunshine. 
