PASSIVE OR ACTIVE MOVEMENTS BY CHLOROPLASTIDS 105 



a stimulus such as contact, or the effect of injury, acts for a prolonged 

 period of time, it may be transmitted for some distance (i to 30 mm. or 

 more) with but little diminished effect, the successive hindrances to its 

 passage being apparently gradually overcome 1 . Thus the infliction of 

 an injury upon a leaf of Elodea or Vallisneria causes a stimulus inducing 

 streaming to pass slowly from cell to cell for some distance. By noting 

 the interval of time between the commencement of streaming in cells at 

 measured distances along the same longitudinal path, the average rate of 

 propagation can be determined. The average rate of propagation along 

 the midrib of Elodea varied from I to 2, mm. per minute at i8C., and 

 from 1-4 to 0-4 mm. per minute at 30 C. In the longer leaf-cells of 

 Vallisneria the rates were 1-2 to 0-5 mm. per minute at i8C., and 2 to 

 0-8 mm. per minute at 30 C. 



In many plants when wounded, the neighbouring cells respond to the 

 injury by a movement of the nucleus to the injured side, and an aggrega- 

 tion of the protoplasm on the same surface. Using this reaction as a test 

 for the reception of the stimulus, Nemec (1. c., p. 35) observed a maximal 

 rate of propagation in root-apices of 0-3 mm. to o-i mm. per minute 

 at room-temperature. This lower rate is probably partly due to the 

 smaller size of the cells. Moreover, the intensity of the stimulus rapidly 

 decreases at a certain distance from the injury, and since the time of 

 reaction undergoes a corresponding increase, the actual rate of propagation 

 is more rapid than it appears to be. Thus with observations commencing 

 three minutes after an injury had been inflicted, a wound-stimulus travelled 

 in the next two minutes 0-55 mm. (0-27 mm. per minute), and in the next 

 seven minutes only 0-33 mm. (0-05 mm. per minute). 



According to Nemec (1. c., p. 49), the rate of propagation is not 

 influenced by light. As regards temperature, no reaction occurs at 2 C. ; 

 at 6 C. a slight reaction is shown after a quarter of an hour for a distance 

 of 0-2 to 0-9 mm. ; at 9 G. the reaction is nearly as rapid as at 18 to 20 C. ; 

 at 42 C. the apparent rate of propagation is nearly the same as at 6 C., 

 and at 35 C. about the same as at 9 C. ; at 43 C. no reaction being 

 shown. The time of reaction and the actual rate of propagation are 

 unfortunately not kept separate in these results, and hence they lose 

 considerably in value. 



SECTION 48. Passive or Active Movements by the Chloroplastids. 



Chloroplastids floating in a stream of plasma can often be seen to 

 rotate obliquely or horizontally on their own axes, or to roll over as they 

 are carried onwards by the moving stream. Jurgensen and also Velten 



1 Cf. Ewart, Ann. du Jard. Bot. de Buitenzorg, 1898, Vol. XV, pp. 190 seq. 



