118 THE CELL 



and hence it moves towards the source of water ; when the re- 

 productive period commences, it shuns moisture, because, at the 

 time when spores are being formed, it diminishes its water supply. 



Many chemical substances attract, whilst others repel plasmodia. 

 If a net of JEihalium, which has spread itself out upon a moist 

 substratum, is brought into contact with a ball of filter paper, 

 which is saturated with an infusion of tan, individual strands of 

 plasma immediately commence to creep towards the nutrient 

 medium. After a few hours all the spaces in the paper ball are 

 filled up with the slime fungus. 



In order to study negative chemotropism, a crystal of common 

 salt or of saltpetre, or a drop of glycerine, may be brought to the 

 edge of the piece of damp filter paper upon which the slime fun- 

 gus has spread itself out. It can then be seen how, as the con- 

 centrated solution of salt or of glycerine gradually creeps along 

 the filter paper, the protoplasm shrinks away from the source of 

 stimulation in ever-widening circles. 



Hence the naked plasmodia, which are so easily destroyed, 

 possess the marvellous property, on the one hand, of avoiding 

 harmful substances, and, on the other, of searching all through 

 the medium in which they are, for substances which are of value 

 to them for purposes of nutrition, and of absorbing them. " For 

 instance, if one of the numerous branches of a plasmodium, by 

 chance comes across a place which is rich in nutriment, an influx 

 of plasma immediately occurs to this favourable spot." 



Pfeffer has Very accurately examined the chemotropism of small, 

 freely motile cells, such as spermatozoa, Bacteria, Flagellata, and 

 Ciliata, in some pioneering investigations that he has made, and 

 by this means has discovered a very simple and ingenious method 

 of investigation. 



He takes some fine glass capillary tubes from 4 to 12 mm. 

 long ; one end of each tube is closed, whilst at the other there is 

 an opening varying in inside diameter from "03 to "15 mm., ac- 

 cording to the size of the organism to be examined. He fills 

 these tubes for about a half or a third of their length with the 

 stimulating substance, there being a space filled with air at the 

 closed end. 



In order to explain their use, we may quote the following ex- 

 periment. Pfeffer has discovered that malic acid has a strong 

 affinity for the antherozoids of Ferns, and that probably it is on 

 this account that it is secreted normally by the archegonia. A 



