IRRITABILITY. 521 



We come now to the consideration of the streaming move- 

 ment of protoplasm, which is commonly designated by the 

 terms Circulation and Rotation. This streaming movement of 

 the protoplasm is made apparent by the granules of foreign 

 substances which are carried along in the current. These 

 terms have been more especially applied to the streaming of 

 the protoplasm of cells enclosed by cell-walls. The difference 

 between them is only superficial, and depends upon the distri- 

 bution of the protoplasm within the cell. When (as in Fig. 3, 

 p. 13) the protoplasm is not confined to the peripheral layer 

 (primordial utricle), and there is a more or less nearly central 

 mass, investing the nucleus, connected by delicate strands, 

 varying from time to time in position and number, with the 

 primordial utricle, the streaming movement can be detected 

 not merely in the peripheral layer, but in the strands in various 

 directions (indicated by the arrows), and is termed Circulation. 

 When, as in the internodal cells of Chara, the protoplasm 

 constitutes only a primordial utricle, the streaming movement 

 follows the contour of the cell, down one side, across the end, 

 up the other side, etc. This is what is termed Rotation. It 

 is not, however, the whole primordial utricle which moves, 

 but an internal layer only, consisting of relatively fluid proto- 

 plasm; the external layer, in which the chlorophyll-corpuscles 

 are imbedded, remains stationary. This moving layer consti- 

 tutes a hollow cylinder, each half of which is moving in the 

 opposite direction to the other, as is well seen when the longi- 

 tudinal line of contact of the two halves is brought into view; 

 the direction of the current on one side of this line is exactly 

 opposite to that on the other. In some cases (Chara, Nitella) 

 this longitudinal line, which is termed the indifferent line, is 

 clearly marked out by the absence of chlorophyll-corpuscles, 

 in the external resting layer, along its course. With regard 

 to the occurrence of the streaming movement of the proto- 

 plasm in plant-cells, it may be pointed out that it is not 

 confined to a few exceptional cases, but that it probably 

 exists, at least for a time, in all plant-cells. This is true 

 particularly of the Circulation, which has been most frequently 

 observed in young cells especially in connexion with the 



