THE VITAL PROPERTIES OF THE CELL 117 



sessed by oxygen explains the fact that in microscopic prepara- 

 tions almost all Bacteria, Flagellata, and Ciliata are found 

 collected together round the edges, or round any air bubbles 

 which may be present in the water. 



Vervvorn describes a most instructive experiment (IV. 40). A 

 large number of Paramrecia are placed in a test-tube, which is 

 filled with water, poor in oxygen. The test-tube is then reversed 

 and placed under mercury. Very soon the movements of the 

 cilia commence to slacken, in consequence of the lack of oxygen. 

 If now a bubble of pure oxygen is introduced through the mer- 

 cury into the test-tube, it will be seen after a few seconds to be 

 surrounded by a thick white envelope of Paramcecia, " which, 

 driven by their thirst for oxygen, throw themselves energetically 

 upon the bubble of this gas." 



2. Liquids. Stahl and Pfeffer have made systematic experi- 

 ments upon the stimulating action of fluid substances. 



Stahl (IV. 35) has again made great use of flowers of tan. 

 Upon this organism even pure water has a stimulating effect, 

 a phenomenon described by Stahl as positive and negative hydro- 

 tropism. If a plasmodium is evenly spread out over a strip of 

 damp filter paper, it is seen, as soon as the paper commences to 

 dry, that the plasmodium makes its way to the dampest parts. If, 

 whilst the drying process is going on, a slide covered with gelatine 

 is held perpendicularly at about two mm. distance above the 

 paper, a few branches are seen to extend themselves upwards to- 

 wards the gelatine, attracted by the water vapour it gives off, 

 until finally they reach it and spread themselves out upon it 

 possibly, during the course of a few hours, the whole plasmodium 

 may transfer itself to the damper surface. When Myxomycetes 

 are about to fructify, negative instead of positive hydrotropism 

 takes place. Under these conditions the plasmodia seek the 

 driest portions of the environment, and withdraw themselves 

 from any damp gelatine or moistened filter paper which may be 

 brought into their neighbourhood. 



These phenomena of hydrotropism. are easily explained by the 

 fact that protoplasm contains a certain quantity of imbibition 

 water, which may fluctuate up to a certain extent, and may even 

 increase or decrease during the development of the cell-body. 

 The more saturated the protoplasm is with water, the more active 

 as a rule are its movements. During the vegetative period the 

 plasmodium of the ^thalium tends to increase its supply of water, 



