CHARACTERISTICS OF PROTOPLASM 5 



by Meyer in Vallisneria, 1827; Robert Brown, 1831, in "Staminal Hairs of 

 Tradescantia." Then came Dujardin's description of the granular streaming 

 in the pseudopodia of Rhizopods and movements in other cells of animal 

 protoplasm (Planarian eggs, von Siebold, 1841; colorless blood-corpuscles, 

 Wharton Jones, 1846). 



The Power of Response to Stimuli, or Irritability. Although the movements 

 of the ameba have been described above as spontaneous, yet they may be in- 

 creased under the action of external agencies which excite them and are there- 

 fore called stimuli, and if the movement has ceased 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. Contact with foreign bodies, gentle pressure, cer- 



FIG. 5. Cell of Tradescantia Drawn at Successive Intervals of two Minutes. The cell- contents 

 consist of a central mass connected by many irregular processes to a peripheral film, the whole 

 forming a vacuolated mass of protoplasm, which is continually changing its shape. (Schofield.) 



tain salts, and electricity produce or increase the movement in the ameba. 

 The protoplasm is, therefore, sensitive or irritable to stimuli, and shows its irri- 

 tability 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 move- 

 ment stops below o C. (32 F. ), and above 40 C. (104 F.); between these 

 two points the movements increase in activity; the optimum temperature is 

 about 37 to 38 C. Exposure to a temperature even below o C. stops the 

 movement of protoplasm, but does not prevent its reappearance if the tem- 

 perature is raised; on the other hand, prolonged exposure to a temperature 

 of over 40 C. kills the protoplasm and causes it to enter into a condition of 

 coagulation or heat rigor. 



b. Mechanical Stimuli. When gently squeezed between a cover and 

 object-glass under proper conditions, a colorless blood-corpuscle contracts 

 and ceases its ameboid movement. 



c. Nerve Influence. By stimulation of the nerves of the frog's cornea, 

 contraction of certain of its branched cells has been produced. 



d. Chemical Stimuli. Water generally stops ameboid movement, and by 

 imbibition causes great swelling and finally bursting of the cells. In some 

 cases, however (myxomycetes), protoplasm can be almost entirely dried up, 

 but remains capable of renewing its movements when again moistened. 



