I0 6 THEORETICAL AND GENERAL 



considered the rolling movement to be an active one, due to the chloro- 

 plastid itself 1 . It is, however, readily explained by the existence of 

 irregularities on the inner surface of the ectoplasm, and by the fact that 

 the velocity in the different layers of endoplasm is by no means precisely 

 uniform. Frequently, however, two neighbouring chloroplastids may 

 appear to be rotating on their axes in opposite directions, one against 

 and the other with the hands of a watch. Berthold 2 considers this to 

 be due to the chloroplastids being inclined in opposite directions to the 

 observer as they roll against the ectoplasm. Seen from above this would 

 give the appearance of rotating in opposite directions 3 . This explanation 

 is borne out by the fact that a chloroplastid, while retaining the same 

 inclination, may be seen to turn over and yet continue to rotate in the 

 same direction as before, whereas if the movement were an active one, 

 it should have been reversed. The latter does, however, actually occur in 

 the chloroplastids of Characeae, according to Hermann (1. c., pp. 30 seq.), 

 who has also observed chloroplastids continue to rotate in a mass of 

 plasma in which streaming had ceased. Similar observations which 

 seemed to indicate an inherent power of movement in chloroplastids were 

 first made by Dutrochet 4 . Chloroplastids lying on the indifferent line 

 may rotate for a time without changing their position, but in every case 

 observed by the writer there was always a possibility that the extreme 

 edge of the chloroplastid was in contact with moving endoplasm, and 

 the temporary adherence to a particular locus is in no wise contradictory 

 to this explanation. 



If the chloroplastids of Characeae have an active power of rotary 

 movement of their own, then they differ in this respect from all other 

 chloroplastids, for these are always passively carried by the streaming 

 plasma. In Elodea, Vallisneria, &c., the dividing line between the moving 

 and non-moving layers may move centrifugally outwards until the whole 

 of the chloroplastids are in movement, and only a thin non-moving film 

 remains in contact with the cell-wall. In Chara and Nitella, however, 

 this never happens ; the outer boundary of the moving layer maintaining 

 a constant position within the chlorophyllous layer, and chloroplastids 

 being relatively rarely drawn into it. Hermann supposes that in passing 

 through the limiting layer 5 the chloroplastid becomes covered by a special 

 film of protoplasm which acts as a motor-mechanism, and endows the 



1 Jurgensen, Studien d. Physiol. Inst. zu Breslau, 1861 ; Velten, Sitzungsb. d. K. Ak. d. Wiss., 

 math, und nat. Klasse, 1876, Bd. LXXIII, I, p. 350 ; Flora, 1876, p. 85. 



2 Protoplasmamechanik, 1886. 



3 This can easily be demonstrated by rolling two blackleads inclined in opposite directions along 

 a walking-stick, and viewing their upper ends. 



* Ann. sci. nat, 1838, T. IX, ii. sen, p. 15. 



5 Hermann regards this layer as the active seat of movement. 



