CHARACTERISTICS OF PROTOPLASM 5 



esses, themselves very little movable, but upon the surface of which freely 

 moving or streaming granules are seen. A gliding movement has also been 

 noticed in certain animal cells; the motile part of the cell being composed of 

 protoplasm bounding a central and more compact mass. By means of the 

 free movement of this layer, the cell may be observed to move along. 



In vegetable cells the protoplasmic movement can be well seen in the 

 hairs of the stinging-nettle and Tradescantia and in the cells of Vallisneria. 

 It is marked by the movement of the granules nearly always embedded in it. 

 For example, if part of a hair of Tradescantia, figures 5 and 6, be viewed 

 under a high magnifying power, streams of protoplasm containing crowds 

 of granules hurrying along, like the foot passengers in a busy street, are 

 seen flowing steadily in definite directions, some coursing round the film 

 which lines the interior of the cell wall, and others flowing toward or away 

 from the irregular mass in the center of the cell cavity. Many of these streams 

 of protoplasm run together into larger ones and are lost in the central mass, 

 and thus ceaseless variations of form are produced. The movement of 

 the protoplasmic granules to or from the periphery is sometimes called 

 vegetable 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 Rosel in 

 1755, as occurring in a small Proteus, probably a large fresh- water ameba. 

 His description was followed twenty years later by Corti's demonstration of 

 the rotation of the cell sap in characeae, and in the earlier part of the last 

 century 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 move- 

 ments of the ameba have been described above as spontaneous, yet they 

 may be increased under the action of external agencies which excite them 

 and are therefore called stimuli. If the movement has ceased for the time, 

 as is the case if the temperature is lowered beyond a certain point, move- 

 ment may be set up again by raising the temperature. Contact with foreign 

 bodies, gentle pressure, certain salts, and electricity produce or increase the 

 movement in the ameba. 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 below o C. (32 F.), and above 40 C. (104 F.); between 

 these two points the movements increase in activity; the optimum tempera- 



