PROTOPLASMIC STREAMING 293 



energy. This will be along as straight or as uniformly curved a path as possible, 

 so that the tendency to eddy currents with their increased resistance to flow is 

 avoided. A spiral path around the long axis of the cell fulfils this condition best 

 when the cell is an elongated cylinder as in Chara and Nitella. When the ends of 

 the cell are rounded the direction of streaming may be parallel to the long axis 

 of the cell 1 . 



The influence exercised by neighbouring cells is shown by the fact that a 

 stimulus awakening or accelerating streaming may radiate to some distance from an 

 injured region. In addition, the planes of streaming in the cortical and medullary 

 cells of the internodes of Chara "* show definite relationships, which may possibly be 

 such as to favour translocation 3 . According to Berthold 4 , there is no constant relation- 

 ship between the direction of streaming in the cells of Elodea and Vallisneria, but as 

 a matter of fact, almost without exception, the direction of streaming is opposed on 

 the two sides of each dividing wall 5 . In the deeper leaf-cells, especially near the 

 midrib, the planes of rotation may intersect at various angles owing to the oblique 

 points of contact of the cells, while in other cases the direction of streaming appears 

 to be primarily determined by the shape of the individual cell. 



SECTION 63. Pulsating Vacuoles. 



Vacuoles may show various changes of shape and volume, and fre- 

 quently fuse as the living cell grows older. When vacuoles periodically 

 diminish and re-enlarge, or disappear and reappear, we speak of contractile 

 or pulsating vacuoles, such as are especially well shown by Infusoria e and 

 by many other Protozoa. They also occur in various Thallophytes and 

 Protophytes, such as most Volvociniae and Flagellatae 7 , a few Palmellaceae 8 , 

 and also in the zoospores of Stigeoclonium, Chaetophora 9 , Ulothrix, 

 Microspora n , and many other Algae, as well as in the zoospores of such 

 Fungi as Saprolegnia 12 and Cystopus 13 , and in the zoospores and plasmodia 



I Ewart, Protoplasmic Streaming in Plants, 1903, p. 35. 



3 A. Braun, 1. c., p. 231. For other cases cf. Hofmeister, 1. c., p. 40. 



3 Hermann, I.e., 1898, p. 13; cf. also Ewart, I.e., p. 34; and The Ascent of Sap in Trees, 

 Phil. Trans., 1905, p. 40. 



4 Berthold, 1. c., p. 121. 5 Ewart, 1. c., 1903, p. 34. 



6 Biitschli, Protozoen, 1880- 8, p. 1411. 



7 Biitschli, I.e., p. 708 ; O. Hertwig, Zelle u. Gewebe, 1893, p. 69, and the literature here 

 quoted; Cohn, Beitr. z. Biol. d. Pflanzen, 1877, Bd. II, p. 117; Klebs, Unters. a. d. hot. Inst. zu 

 Tubingen, 1883, Bd. I, p. 246; Senn, in Engler's Natiirl. Pflanzenfamilien, 1900, T. I, Abth. i, 

 p. 101. 



3 Cienkowski, Bot. Ztg., 1865, p. 22 ; 1876, p. 70. g Id., 1876, p. 70. 



10 Strasburger, Zellbildung u. Zelltheilung, 1875, p. 157; Dodel, Bot. Ztg., 1876, p. 183. 



II Maupas, Compt. rend., 1876, T. LXXXII, p. 1,451. See also Falkenberg in Schenk's Hand- 

 buch d. Botanik, 1882, Bd. n, p. 194; Hofmeister, Pflanzenzelle, 1867, p. 12 ; Woronin, Bot. Ztg., 

 1880, p. 628 (Chromophyton). 



12 Rothert, Cohn's Beitr. z. Biol., 1892, Bd. v, p. 323. 



13 De Bary, Ber. d. nat. Ges. zu Freiburg, 1860, p. 8. 



