100 THE CELL 



propel themselves by means of cilia or flagella, such as Flagel- 

 lata, Ciliata, the swarm-spores of Algce, etc., prefer to collect 

 either on that side of the cultivation dish which is nearest the 

 window, or on the one which is away from it. 



This may be easily proved by means of a simple experiment 

 described by Nageli (III. 16). A piece of glass tubing three feet 

 in length is filled with water containing green swarm-spores of 

 Algce (tetras pores), and is placed perpendicularly. Then, if the 

 upper part of the tube is covered with black paper, and light is 

 allowed to fall upon the lower portion, it is seen after a few hours 

 that all the spores have collected in this lower portion, leaving 

 the upper part colourless. If now the upper portion is uncovered, 

 and the paper is transferred to the lower part, all the swarm- 

 spores ascend the tube, and collect on the surface of the water. 



Euglena viridis is exceedingly sensitive to light (Fig. 44 A, 

 IV. 8). If a drop of water containing Euglence is placed upon a 

 slide, and only a small portion of it is illuminated, all the 

 individuals collect in this area, which, to quote an expression 

 of Engelmann's, acts like a trap. This organism is especially 

 interesting, because the perception of light is restricted to a 

 definite portion of the body. Each Euglena consists of two 

 portions, a large posterior one containing chlorophyll, and a 

 colourless anterior, flagella-bearing one, in which there is a red 

 pigment spot. Now, it is only when this anterior portion comes 

 into contact with light, or is placed in shadow, that the organism 

 is seen to react by altering the direction of its movements 

 (Engelmann). Hence, in this case, a certain part of the body 

 functions to a certain extent as an eye. 



Stahl (IV. 34) and Strasburger (IV. 37) have investigated most 

 fully the action of light upon swarm-spores. The former sums up 

 his results in the following words : " Light effects an alteration 

 in the direction of the movements of swarm-spores by causing 

 them to make their longitudinal axes coincide approximately with 

 the light. The colourless flagellated end may be directed either 

 towards or away from the source of light. Either position may 

 become exchanged for the other under otherwise unaltered 

 external conditions, and, indeed, this occurs at very different 

 degrees of light intensity. The intensity has the greatest influence 

 over relative positions. When the light is very intense, the 

 anterior end is directed away from the source; when it is less 

 strong, the swarm-spores move towards the light." 



