338 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



geotactic as it actually is. Such forms as Euglena viridis and Chlamydo- 

 monas pulvisculus only have their negative geotaxis mechanically overcome 

 when they are subjected to centrifugal forces eight times greater than that 

 of gravity, while the limit of geotactic perception seems to lie between 

 0-2 to 0-3 g., according to Schwarz. It is, however, uncertain whether 

 the perception arises from the movements of the denser particles in the 

 organism or is aroused by differences of pressure in the medium. 



SECTION 73. Diffuse Chemical Actions. 



Locomotion and intercellular movement, like all forms of vital activity, 

 are dependent upon metabolism, and hence cease sooner or later when the 

 latter is partially or completely suppressed. The same is the case when 

 oxygen is removed from an aerobic organism, although growth and move- 

 ment may be maintained under relatively low partial pressures of oxygen. 

 On the other hand, the access of air to obligate anaerobes, as well as 

 a sufficient increase of the partial pressure of oxygen upon aerobes, retards 



and ultimately inhibits their 

 powers of growth and move- 

 ment. Among certain facul- 

 tatively anaerobic bacteria, 



FIG. 57. Median section of gas-chamber (reduced). however, the absence of OXygen 



causes movement to cease 



although growth continues 1 . The power of movement is, however, retained, 

 and is at once shown when oxygen is admitted. Possibly in other cases 

 the removal of oxygen may produce a stoppage of growth before move- 

 ment, and more especially protoplasmic streaming, have ceased. 



The withdrawal of nutriment or even of a single essential constituent 

 must sooner or later retard or stop movement, although streaming may 

 continue in starving plants almost until death ensues 2 . This is the case 

 in cells of Chara and Nitella, whereas in those of Elodea and Vallisneria 

 a long period may elapse between the cessation of streaming and the 

 permanent loss of vitality. On the other hand, cells packed with food- 

 materials show no streaming, and the latter is not shown until the cells are 

 partially emptied 3 . This in part arises from the decreased resistance 

 coupled with the great activity of the cell during translocation, and 

 naturally also the addition of food-materials accelerates streaming in 

 starved cells. 



1 Ritter, Flora, 1899, p. 329. 



a Kiihne, Zeitschr. f. Biologic, 1898, N. F., Bd. xvni, p. 85 ; Ritter, 1. c., p. 355; Ewart. 

 Protoplasmic Streaming in Plants, 1903, p. 76 ; Wallengren, Zeitsch. f. allgem. Physiol., 1902, Bd. I, 

 p. 67. 



3 Ewart, 1. c. 



