en. n. ] 



IRRITABILITY OF PROTOPLASM 



13 



central mass, and thus ceaseless variations of form are produced. 

 The movement of the protoplasmic granules to or from the peri- 

 phery is called circulation, whereas the movement of the protoplasm 

 round the interior of the cell is called rotation. 



The first account of the movement of protoplasm was given by 

 Kdsel in 1755, as occurring in a small 

 Proteus, probably a large freshwater 

 amoeba. His description was followed 

 twenty years later by Corti's demonstra- 

 tion of the rotation of the cell sap in 

 Characese, and in the earlier part of last 

 century by Meyer in Yallisneria, 1827, 

 and by Eobert Brown, 1831, in " Staminal 

 Hairs of Tradescantia." Then came Du- 

 jardin's description of the granular stream- 

 ing in the pseudopodia of Ehizopods; 

 movements in other animal cells were 

 described somewhat later (Planarian eggs, 

 v. Siebold, 1841 ; colourless blood-cor- 

 puscles, Wharton Jones, 1846). 



There is no doubt that protoplasmic 

 movement is essentially the same thing 

 in both animal and vegetable cells. But 

 in vegetable cells the cell-wall obliges 

 the movement to occur in the interior, 

 while in the naked animal cells the move- 

 ment results in an external change of 

 form. 



Although the movements of amoeboid cells may be loosely de- 

 scribed as spontaneous, yet they are produced and increased under 

 the action of external agencies which excite them, and which are 

 therefore 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. 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 45 C. (113 F.); between these two 

 points the movements increase in activity ; the optimum temperature 

 is about 37 to 38 C. Though cold stops the movement of proto- 

 plasm, exposure to a temperature even below C. does not prevent 



FIG. 14. Cells from the staminal 

 hairs of Tradescantia. A, fresli 

 in water ; B, the same cell after 

 slight electrical stimulation ; 

 a, ft, region of stimulation ; 

 c, d, clumps and knobs of con- 

 tracted protoplasm. (Kiihne.) 



