PEOTOPLASMIO MOVEMENT. 403 



together, as appears to be almost universally the case with 

 vegetable cells containing moving protoplasm, at the short 

 transverse walls (ends of the cells), and the whole mass contracts, 

 so as to expose the least superficies, the process thus corre- 

 sponding with the formation of spherical bodies by naked 

 protoplasm when subjected to excitation. Briicke^ observed 

 upon the stinging hairs of Urtica that after short powerful 

 excitations threads of protoplasm with pointed ends and swel- 

 lings in the middle, or knob-like, club-shaped processes stood 

 out from the primordial utricle, being afterwards withdrawn 

 again. 



2. Thermal Stimuli. 



Both positive and negative alterations of temperature may 

 act as stimuli, and produce results similar or identical with 

 those produced by electrical stimuli. This, of course, only 

 when the alterations fall within the temperature range of 

 contractility. In this case also the effect is the more pro* 

 nounced and lasting the more rapid and extensive the change 

 of temperature. Negative changes (always ?) appear to act to 

 a specifically greater extent than positive. 



If after a change the temperature remains constant, the 

 movements gradually assume the condition which they would 

 have assumed by quite gradual warming or cooling, as the case 

 might be, to a corresponding degree. 



In Char a, which exhibited a moderately rapid rotation in 

 water at 7°, Dutrochet '^ observed that the movements completely 

 ceased in four to five minutes when plunged into water at 32°. 

 After some hours in water at 32°, the movements commenced 

 again, and in two more hours had become quite active once 

 more. Replaced in water at 7° the movements were again de- 

 stroyed in four minutes, but recommenced quite slowly after 

 half an hour at this temperature. After somewhat slower 



' E. Brlicke, " Das Verbalten der sog. Protoplasma-strome ia den Brenn- 

 haaren von Urtica urens," 'Sitzsher. d. Wiener Akad.,' xlvi, p. 2, 1863. 

 M. Schultze, 'Das Protoplasma, &c.,' p. 45. 



- Dutrochet, 'Compt. rend.,' ii, p. 777, 1837. 



