294 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



of Myxomycetes 1 . No typical pulsating vacuoles have hitherto been 

 detected in the cells of plants above the Thallophyta, but nevertheless 

 transition forms occur between normal and pulsating vacuoles. All vacuoles 

 are formed in the same way by the protoplasm, and many normal vacuoles 

 undergo pronounced but slow changes of volume 2 , which are in some cases 

 mechanically produced by protoplasmic streaming or surface-tension action. 

 In fact there is probably no vacuole whose size and shape are constant, 

 while various external agencies may progressively reduce and ultimately 

 inhibit the rhythmic activity of pulsating vacuoles. Periodic changes of 

 concentration in the external medium involve corresponding periodic 

 changes of volume in the vacuole, so that the normal progress of 

 metabolism is bound to influence the size of the vacuole, although such 

 purely mechanical actions may be controlled and regulated by the 

 protoplast within certain limits. Thus a rise of concentration in the 

 external medium will exercise no influence upon the size of the vacuole, if 

 the osmotic concentration of the cell-sap is proportionately increased, and 

 when the vacuole is very small, a fall of the surface-tension of the vacuolar 

 membrane would be almost equally effective in balancing the increased 

 external pressure. 



In Closterium, and a few other Desmids, the vacuole occurring at each 

 pole becomes smaller when the direction of streaming of the protoplasm is 

 towards that end, and it regains its original size with the periodic reversal 

 of the stream 3 . In the hyphae of Ascobolus and other Fungi, periodic 

 alterations in the volume of the vacuoles produce to-and-fro streaming 

 movements in the protoplasm, while the periodic movements of the leaves and 

 leaflets of Desmodium and Trifolium and of other plants involve rhythmic 

 contraction and dilation of the cells and hence also of the vacuoles. The 

 same occurs during every stimulatory movement of the filaments of Cynareae 

 and the pulvini of Mimosa, for this involves a considerable escape of water 

 from the cell and its subsequent reabsorption. 



Pulsating vacuoles are always small, they usually maintain the same 

 locus and commonly not more than one to three are present in plant-cells 4 . 

 The plasmodia of Myxomycetes have, however, numerous pulsating vacuoles 

 which may be present not only in the ectoplasm but also in the streaming 

 endoplasm. Most Volvocineae have two pulsating vacuoles, but Vofoox has 

 only one, and Chlorogonium has numerous contractile vacuoles 5 . According 



1 De Bary, Mycetozoen, 1864, pp. 41, Si ; Cienkowski, Jahrb. f. wiss. Bot., 1863, Bd. in, 

 p. 329; Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, pp. 192, 219, 336. On Amoeba 

 cf. Biitschli, Protozoen, 1880-8, p. 105; Rhumbler, Archiv f. Entwickelungsmechanik, 1878, Bd. vn, 

 p. 256. 



2 Pfeffer, I.e., p. 257; Butschli, I.e., pp. 1412, 1435; Rhumbler, 1. c. 



3 De Bary, Unters. ii. d. Familie d. Conjugaten ; Schumann, Flora, 1875, p. 66; A. Fischer, 

 Jahrb. f. wiss. Bot., 1884, Bd. xiv, p. 142. 



* Butschli, 1. c., p. 708. 5 Massart, Bull, de 1'Acad. royale de Belgique, 1901, p. too. 



