TACTIC RESPONSE TO TROPIC STIMULI 313 



equally excitable. A phobic response would, however, still be possible, for 

 the time of a rotation is longer than the latent period of stimulation. 



Individual differences appear to be of commoner occurrence among 

 lower than among higher organisms; and, although critical researches are 

 wanting, it appears that in the case of many Bacteria and Infusoria the 

 irritability may vary according to the cultural conditions, so that a 

 particular species may react at one time strongly, at another feebly or 

 not at all to a particular agency l . It is even possible that races may be 

 bred which are devoid of an irritability possessed by the common stock. 



SECTION 67. The Influence of Temperature. 



The maxima and minima for locomotion and streaming approximate 

 to those for growth, although plants may be found able to grow at 

 temperatures which do not permit of streaming or locomotory activity. 

 Both forms of movement may, like growth, continue for a time at a supra- 

 maximal or supraminimal temperature which ultimately proves fatal. 

 Zopf 2 observed, for instance, that Bacterium vernicosum, whose maximal 

 temperature for growth is 45 to 46 C, continues to move for a time at 

 50 to 52 C. Streaming may still be present in the cells of Chara, Nitella, 

 and Elodea after ten minutes' exposure to 50 C., and in Elodea after an 

 even longer exposure to 55 C. 3 The determination of the optimum 

 points is rendered difficult by the fact that even in the absence of any 

 shock-effect the velocity assumed at high but not fatal temperatures is 

 always more rapid than it becomes after prolonged exposure, as the cell 

 becomes accommodated or fatigued 4 . On the other hand, after prolonged 

 exposure to low temperatures a moderate rise may take some time to 

 produce its full effect. In addition, the tone may be modified in other 

 ways. Thus, according to Josing 5 , streaming ceases within two minutes 

 at 45 C., but not till after twenty minutes' exposure to this temperature in 

 water containing 0-25 per cent, of ether 6 . Individual variations are also 

 shown, for Ewart found that in some leaf-cells of Vallisneria streaming 

 was retarded beyond 35 C., but in other cases not until 45 C. was reached, 

 and an equally low optimum was obtained when the temperature was very 

 gradually raised. Moderate rises of temperature influence the velocity 

 of streaming in two ways either by lowering the viscosity of the endoplasm 

 or by increasing either the total amount of energy generated or the 



1 Rothert, Flora, 1901, p. 417. 



2 Zopf, Beitr. z. Physiol. u. Morphol. niederer Organismen, 1892, Bd. I, p. 66. 



8 Ewart, Protoplasmic Streaming in Plants, 1903, p. 59. * Id., p. 62. 



5 Josing, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 217. 



6 [Some doubt attaches to these results, for streaming may also continue for twenty minutes in 

 the leaf-cells of Vallisneria spiralis at 45 C. in the absence of any ether. Cf. Ewart, 1. c., p. 65.] 



