MOVEMENTS OF IRRITATION. 435 



Light of any great- intensity, as it appears, e.g., diffuse daylight, 

 plasmodia always avoid. If we expose pieces of tan traversed by 

 plasmodia to the light, in. the moisture-saturated chamber, the 

 plasmodia retract into the tan. Similarly plasmodia spread out 

 on sheets of glass, when exposed to the light, seek places where 

 the intensity of the light is least, i.e. shaded places. The plas- 

 modia are easily transferred to sheets of glass, even object-glasses 

 for example, as follows : A beaker is filled with water, into 

 which is brought one end of a strip of blotting-paper whose 

 width is somewhat less than that of our object-glass. The other 

 end of the blotting-paper is laid on the face of an object-glass 

 standing vertically beside the beaker. The whole arrangement 

 is placed on a layer of sand, which takes up the water running 

 down from the object-glass. At the base of the object-glass is 

 next put a piece of tan bearing a plasmodium, the tan being of 

 course leaned against that face of the glass down which the water 

 is running. We finally cover with a bell-glass, and exclude the 

 light. The plasmodium, in virtue of its rheotropism, now creeps 

 on to the object-glass, and spreads out on it. If for the sake of 

 closer examination we desire to have plasmodia on the slide under 

 a cover-glass, it is a good plan to fix the cover-glass on the side 

 of the slide down which the current of water is flowing, by 

 means of bits of wax placed under the four corners. It is then 

 possible for branches of the plasmodia to creep under the cover- 

 glass, and here spread out forming delicate threads. The accu- 

 mulation of the plasmodia on the slides does not always proceed 

 with the same rapidity ; generally it is sure to take place in the 

 course of half a day. 



It is of very special interest to study exactly the attractions 

 and repulsions exerted by different substances on the direction of 

 movement of plasmodia. We employ for the experiments plas- 

 modia coaxed on to blotting-paper by means of a current of 

 water, or the necessary material is obtained from Aethalium 

 sclerotia, by putting them on a damp surface (several layers of 

 blotting-paper saturated with water). I obtained in this last 

 way beautiful plasmodia, and when they had been made fairly 

 hungry by being kept for a considerable time under a bell-glass, 

 a, condition favourable for the experiment, small balls of blotting- 

 paper saturated with tan extract were laid on them. The sub- 

 stances present in the tan extract exert an attractive influence on 

 the plasmodia, and hence, even after a few hours, the paper balls 



