PROTOPLASMIC STREAMING 287 



fact that the thickness of the non-streaming ectoplasmic layer is variable 

 and that it is often extremely thin, its immotility is only relative, for it 

 undergoes changes of shape during plasmolysis and amoeboid movement. 



In addition to the non-moving external layer, large or smaller portions 

 of the general protoplasm may be temporarily or permanently in relative 

 rest. A transitory period of quiescence occurs between each rhythmic 

 reversal of circulatory streaming, and in some cases streaming may be 

 shown only along isolated bands. Even when streaming is general, a 

 narrow or even a broad indifferent line of rest is interposed between the 

 ascending and descending streams, and this line is characterized in Chara 

 and Nitella by the absence of chloroplastids. The dividing line between 

 neighbouring streams may, however, be of merely theoretical dimensions 

 without any signs of mutual disturbance, obliquely moving chloroplastids 

 being repelled from the neutral line as though an invisible elastic limiting 

 membrane separated the two moving layers 1 . 



Local streaming or gliding movements will naturally be produced 

 whenever the motory energy is localized, or is insufficient in amount to 

 produce complete streaming. Temporary local sliding movements are in 

 fact shown when young cells are acquiring the full power of streaming, 

 and also when streaming is recommencing in anaesthetized or partially 

 disorganized cells 2 . These movements have been termed digression move- 

 ments by Wigand 3 , and they often consist merely of to-and-fro movements 

 of individual particles of protoplasm. 



The streaming endoplasm may carry with it various inactive suspended 

 bodies such as crystals, starch-grains, oil-drops, and vacuoles, and even 

 the nucleus or nuclei and chloroplastids may be passively carried with the 

 stream. At the same time the latter may possess a feeble tendency to 

 slow locomotion, which is, however, imperceptible when they are rapidly 

 carried round the cell, but which becomes perceptible when they are 

 embedded in resting protoplasm. Apart from their slow amoeboid changes 

 of shape, it is doubtful whether the nucleus and chloroplastids possess any 

 well-defined powers of locomotion. Naturally when a non-motile body 

 is in contact with a stationary layer on one side it will move more slowly 

 and tend to acquire a rotary movement or may even temporarily move 

 in the opposite direction as compared with particles surrounded on all sides 

 by streaming protoplasm 4 . Hormann 5 has suggested that the chloroplastids 



1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 108. 



2 Cf. Nageli, Pflanzenphysiol. Unters., 1855, Heft i P- 495 Beitrage z. wiss. Bot., 1860, Heft ii, 

 pp. 10, 84; Velten, Eot. Ztg., 1872, p. 651. 



3 Bot. Hefte 1885, Heft i, p. 180. 



* Goppert und Cohn, Bot. Ztg., 1849, p. 698 ; Nageli, Beitr. z. wiss. Bot., 1860, Heft ii, p. 66 ; 

 Velten, Activ oder passiv? Oesterr. Bot. Zeitschrift, 1876, Nr. 3; Eerthold, 1. c., pp. 118, 150; 

 Wigand, I.e., p. 195. 



5 Hormann, Studien iiber die Protoplasmastromung Lei d. Characeen, 1898, p. 24. 



