THE VITAL PROPERTIES OF THE CELL 115 



solution, and rinsed with clean water, was placed in pure sugar 

 solution, he found that fermentation soon occurred; hence the 

 yeast had recovered its power of converting sugar into alcohol 

 and carbon dioxide, this power having, by the action of the 

 chloroform and ether, been temporarily suspended. 



In a similar manner the functions which the chlorophyll per- 

 forms in plants, and the dependent process of giving off oxygen in 

 the sunlight, may be arrested by means of chloroform (Claude 

 Bernard). 



6. Second Group of Experiments. Chemical Stimuli 

 which come into contact with the cell-body at one spot 

 only. Very interesting and varying phenomena are produced 

 when chemical substances, instead of coming into contact with the 

 body all round, only impinge upon it, at a definite fixed point. 

 Such stimuli may produce changes in form, and movements in a 

 definite direction, which phenomena have been classed under the 

 name of Chemotropism (Chemotaxis) . 



Chemotropic movements may be directed towards the stimulating 

 source, or, on the contrary, away from it. In the first case the chemi- 

 cal substance is said to attract, and in the second to repel, the 

 protoplasmic body. This depends partly upon the chemical 

 nature of the substance, partly upon the individual properties of 

 the special kind of plasma, and, finally upon the degree of conden- 

 sation of the chemical substance. A substance, which when 

 dilute may attract, may repel when the solution is strong. Here, 

 as with strong and weak light, special differences are present. 

 Just as heliotropism may be positive or negative, so may chemotro- 

 pism be positive or negative. 



We will first examine the action of gases, and next that of 

 solutions ; at the same time we will become acquainted with a very 

 ingenious method of investigation, for which we must especially 

 thank the botanist Pfeffer (IV. 26). 



1. (rases. Oxygen has great attractive powers for freely 

 moving cells, as has been shown by the experiments of Stahl, 

 Engelmann, and Verworn. 



Stahl has made experiments upon the plasmodia of JEthalium 

 septicum (IV. 35). He half filled a glass cylinder with thoroughly 

 boiled water, which, in order to exclude the air, he covered with 

 a very thin layer of oil. He then took a strip of filter paper, over 

 which a plasmodium had extended itself, and placed it along the 

 side of the cylinder in such a manner that one half of it was. 



