THE INFLUENCE OF THE EXTERNAL CONDITIONS 307 



retard or inhibit movement may accelerate it when dilute. The fact that 

 light causes certain zoospores, and meat extract those of Saprolegnia, to 

 come earlier to rest * is due to the shortening of the period of development 

 by these agencies. Whether the similar influence of magnetic forces is also 

 of this character is, however, uncertain 2 . 



The existence of a power of rapid locomotion permits the shock-effects 

 of sudden changes to become more readily perceptible. The sudden 

 application of fatally injurious conditions often causes specially active 

 irregular locomotion which reminds one of the spasmodic struggles of a 

 poisoned or asphyxiating animal. Naturally shock- reactions are not always 

 equally pronounced, and are not shown in all cases and with all agencies. 

 Changes of temperature, of illumination, and of concentration, injuries 

 and transitory anaesthetization, as well as many other agencies, may excite 

 or accelerate protoplasmic streaming, and in some cases when once aroused, 

 especially as the result of injury, it may persist until death. The direct 

 action of a sudden change upon existent streaming is usually to cause 

 a temporary retardation or even stoppage ; but in some cases, especially 

 with moderate rises of temperature, the velocity is temporarily accelerated 

 beyond the value it ultimately assumes. Injurious external agencies, 

 especially when suddenly applied, usually cause a contraction of amoeboid 

 protoplasts to the spheroidal shape, but may occasionally increase the 

 amoeboid activity. 



Contact or the change to another medium causes the cilia of Chlamydo- 

 monas to straighten suddenly, and so produces a backward movement of the 

 organism into the homogeneous medium, in which the ciliary and locomotory 

 activity is resumed in one or more seconds 3 . A similar shock-movement 

 is produced in Bacterium photometricum by sudden decreases of illumina- 

 tion, and this may cause it to move ten to twenty times its length backwards 

 when it comes to the edge of an illuminated area to which it is, therefore, 

 restricted. The transit from a concentrated to a more dilute solution 

 produces a similar backward movement in many Bacteria, Infusoria, and 

 Flagellatae, so that the organisms collect in the more concentrated medium. 

 It is, however, not known whether this shock-movement is accompanied by 

 a temporary cessation of the ciliary activity, although, according to Fischer, 

 sudden changes of concentration do actually cause a temporary inhibition 

 of the ciliary movement 4 . All motile organisms do not show shock-reactions 

 of this character, and an organism sensitive to one form of shock may be 

 insensitive to others. The shock-movement of Bacterium photometricum is 

 produced only by the transit from light to darkness, and not by the reverse, 



1 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, p. 467 ; Rothert, Flora, 1901, p. 374. 



2 Ewart, Protoplasmic Streaming in Plants, 1903, p. 52. 

 s Pfeffer, 1. c, p. 444. 



* A. Fischer, Jahrb. f. wiss. Bot., 1895, Bd. xxvil, p. 76. 



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