284 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



is constant, and is such as to be in opposite directions on the two sides 

 of the dividing-wall between each pair of contiguous cells. Occasionally, 

 however, as the result of injury or death to neighbouring cells, the direction 

 of streaming may be altered or reversed \ and during circulatory streaming 

 reversals or changes of direction may occur at longer or shorter periodic 

 intervals 2 . 



In comparison with the movement of zoospores, protoplasmic streaming 

 is slow, for the maximum rate observed in the plasmodium of Didymium 

 serpula is 10 mm. per minute, while in the cells of Vallisneria and of 

 Elodea the streaming protoplasm usually does not cover more than 

 i5 mm. per minute 3 , and in the internodal cells of Chara and Nitella 

 rarely exceeds a to 3 mm. per minute 4 . Arthur 5 observed a velocity 

 of 3-3 mm. per minute in the hyphae of Rhizopus nigricans. In the 

 case of Chara and Nitella, where the streaming endoplasm is comparatively 

 thick, the velocity varies in different parts, being most rapid in the layers 

 just outside the median line of the endoplasm, falling thence abruptly to 

 nil against the ectoplasm, and diminishing gradually towards the cell-sap, 

 the outer layers of which move with the protoplasm. In addition, gravity 

 affects to a very slight extent the speed of floating particles of varying 

 density, accelerating or retarding their velocity according to whether the 

 streaming is upwards or downwards 6 . In regard to the size of the cell, 

 however, the movement is comparatively rapid, for four to six rotations 

 may be performed per minute by the streaming protoplasm in the cells 

 of Elodea and Vallisneria. 



Although the protoplasm is never absolutely at rest, numerous cells 

 do not show any perceptible streaming, nor can any be awakened in 

 them. Frequently, however, rapid streaming can be excited in inactive 

 cells by injury or by treatment with various chemical substances. This 

 applies to the leaf-cells of Vallisneria and to the leaf-parenchyma cells 

 of Elodea, whereas in the leaf-hairs of Cucurbita and Urtica and in the 

 staminal hairs of Tradescantia as well as usually along the midrib of Elodea 

 streaming appears under normal conditions. Hauptfleisch attempts to 

 distinguish between the streaming normally present and that excited by 

 stimuli as 'primary' and 'secondary' streaming, but the distinction is 

 a purely artificial one and cannot be applied in all cases 7 . In any case 



1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 34. 



3 On the distribution and special peculiarities of streaming cf. Hofmeister, Pflanzenzelle, 1867, 

 p. 48 ; Velten, Bot. Ztg., 1872, p. 672 ; Wigand, Bot. Hefte, 1885, Heft i, p. 169 ; Berthold, Proto- 

 plasmamechanik, 1886, p. 119; Janse, Jahrb. f. wiss. Bot., 1890, Bd. XXI, p. 198 (Caukrpd)\ 

 Ternetz, ibid., 1900, Bd. xxxv, p. 273 (Ascobolus}. 



3 Hofmeister, 1. c., p. 48. * Cf. Ewart, 1. c., pp. 24, 25, 63, 65. 



* Arthur, Annals of Botany, 1897, Bd. XI, p. 493. 



6 Cf. Ewart, I.e., pp. 23, 113. 



7 Cf. Ewart, pp. 4, 75 ; Hauptfleisch, Jahrb. f. wiss. Bot., 1892, xxiv, pp. 190-200. 



