408 PROF. TH. W. ENGBLMANN. 



after the preparation has been left quiet for a time.^ This also 

 holds good according to some observations, for Diatoms and 

 Oscillatoria. 



5. Chemical Stimuli. 



Sudden chemical reactions produce the same effects as are 

 observed after electrical excitations, but complicated and in- 

 terrupting accessory phenomena are often present (shrinking, 

 swelling, coagulation, &c.). So only in very few cases is it 

 possible to observe the special effects of the excitation as 

 evinced by the form and temporary structure. 



Sudden changes in the amount of the contained water may 

 act as stimuli. 



Dutrochet 2 placed Chara in salt solution of more than 1 per 

 cent. After four minutes the movements stopped, and after 

 eight minutes more recommenced again. Gradually this 

 became very rapid, and continued for ten days. A similar 

 preparation in which the movement had been stopped, and 

 after ten hours^ immersion had become quite active again, was 

 plunged into pure water at the same temperature, and this 

 caused a cessation of the movement in four minutes which 

 lasted for five minutes. Hofmeister^ says, after observations 

 on Chara, Vallisneria, Hydrocharis, and Tradescantia : — " The 

 treatment of a cell which contains streaming protoplasm with 

 a watery solution of a substance not directly injurious to the 

 vital activity of the plant when of a concentration sufficient to 

 cause the sudden drawing together of the protoplasmic contents 

 of the cell, stops all flowing movement of the protoplasm for a 

 short time during the contraction of the protoplasmic contents, 

 but a rapid streaming of the undivided peripheral layer soon 

 commences again." 



After the sudden dilution of a solution in which a cell with 

 easily permeable walls (e.g. leaf-cell of Vallisneria, root 



' Hofmeister, 1. c, p. 50, 



2 Dutrochet, ' Compt. rend.,' ii, pp. 781, 782, 1837. 

 * Hofmeister, loc. cit., p. 52 ; cp. also ibid., p. 27, for corresponding obser- 

 vations on Didimene serpula. 



