174 Wager: Movements of Aquatic Micro-Organisms. 
other conditions the non-motile state is a persistent one, but 
Euglena can pass from one state to the other with facility 
should the conditions become unfavourable. We find for 
example, that the non-motile condition is induced in the 
presence of light and air (oxygen), that is, under such con- 
ditions as allow of a vegetative existence by means of its 
chlorophyll. If the conditions are unfavourable for the proper 
functioning of the chlorophyll the organism has to subsist 
upon its organic food supply, and under such conditions it 
tends to remain motile. 
Euglena, like many other motile chlorophyll containing 
organisms, is very sensitive to the light. Our knowledge of 
this is due mainly to the investigations of Strasburger* * * § and 
Engelmannf, but important contributions to our knowledge 
of the more intimate details of the phenomena have been 
made by numerous other investigators among whom may be 
mentioned Stahl, J Loeb,§ Jennings, || and Mast, if 
Generally it has been found that Euglenae are attracted by 
light of moderate intensity and are repelled by a strong light. 
The degree of sensitiveness which they exhibit in this respect 
varies considerably, however, even in individuals of the same 
culture. The effects produced are very striking and can 
very easily be demonstrated. 
They are strongly attracted by a bright light, such as that 
of a gas-flame or incandescent burner focussed, by means of a 
substage condenser, upon a microscope-slide on which a drop 
of water containing Euglenae has been placed. Such a spot 
of light will attract, in the space of about one minute, the 
majority of the cells in the field of the microscope, as seen by 
a one-inch objective ; and in two minutes only very few will 
be found outside the light area. If a large number of cells 
are present, they will form a seething mass in the light space, 
perhaps two or three layers deep. On turning down the con- 
denser so as to get the light spread evenly over the whole field 
of the microscope, they begin at once to move away rapidly in 
all directions, and in one or two minutes are found in all parts 
of the field. The intensity of the light appears to be a factor 
in the precision with which the movements are effected. In 
light of low intensity the path taken by the Euglenae in the 
* ‘ Wirkung ties Lichtes und der W'arme auf Schwarmsporen.’ Jena 
Zeitsclir. xii., 1878. 
f ‘ I’eber Licht und Farbenperception niederster Organismen.’ 
Pfliiger’s Archiv, Bd. 29, 1882 (see J.R.M.S. 1883, page 81). 
J ‘ Ueber den Einfluss von Richtung und Starke der Beleuchtung auf 
einige Bewegungserscheinungen im Pflanzcnreichc.” Bot. Zeitung, 
1883. 
§ ‘ The Dyamics of Living Matter.’ New York, 1906. 
jl ‘ Behaviour of the Lower Organisms.’ New York, 190G. 
^ ‘ Light and the Behaviour of Organisms.’ New York, 1911. 
Naturalist, 
