CH. II.] 



IRRITABILITY OF PROTOPLASM. 



for the time, as is the case if the temperature is lowered beyond a 

 certain point, movement may be set up by raising the temperature. 

 Again, contact with foreign bodies, gentle pressure, certain salts, 

 and electricity, produce or increase the movement in the amoeba. 

 The protoplasm is, therefore, sensitive or irritable to stimuli, and 

 shows its irritability by movement or contraction of its mass. 



The effects of some of these stimuli may be thus further 

 detailed : 



a. Changes of temperature. Moderate heat acts as a stimulant : 

 the movement stops when the temperature is lowered near the 

 freezing point or raised above 40 C. 



(104 F.) ; between these two points 

 the movements increase in activity ; 

 the optimum temperature is about 

 3 7 to 38 C. Though cold stops the 

 movement of protoplasm, exposure 

 to a temperature even below o C. 

 does not prevent its reappearance if 

 the temperature is raised ; on the 

 other hand, prolonged exposure to a 

 temperature of over 40 C. altogether 

 kills the protoplasm and causes it 

 to enter into a condition of coagula- 

 tion or heat rigor. We have already 

 seen that proteids, the most abun- 

 dant constituents of protoplasm, are 

 coagulated by heat. 



b. Chemical stimuli. Distilled 

 water first stimulates then stops 

 amoeboid movement, for by imbibi- 

 tion it causes great swelling and 

 tii i. illy bursting of the cells. In 

 some cases, however (myxomycetes), 



protoplasm can be almost entirely dried up, but remains capable of 

 renewing its movement when again moistened. Dilute salt solution 

 and very dilute alkalis stimulate the movements temporarily. 

 Acids or strong alkalis permanently stop the movements : ether, 

 chloroform, veratrine and quinine also stop it for a time. 



Movement is suspended in an atmosphere of hydrogen or 

 carbonic acid and resumed on the admission of air or oxygen ; 

 complete withdrawal of oxygen will after a time kill protoplasm. 



c. Electrical. Weak currents stimulate the movement, while 

 strong currents cause the cells to assume a spherical form and to 

 become motionless. 



Fig. 16. Cells from the stamina 

 hairs of Tradescantia. A, fresh in 

 water ; B, the same cell after slight 

 electrical stimulation ; a, b, region 

 < >f stimulation ; c, d, clumps and 

 nobs of contracted protoplasm. 

 (Ktthne.) 



