522 LECTURE XX. 



formative processes. From the nature of the case, Rotation 

 can only be observed in full-grown cells. Nor is the streaming 

 movement confined to protoplasm enclosed in a cell-wall; 

 it is strikingly exhibited by the plasmodia of the Myxomy- 

 cetes. 



The last kind of movement to be considered under this 

 head is that of pulsating vacuoles. These are small more or 

 less nearly spherical cavities which make their appearance in 

 the protoplasm, and then suddenly disappear. In the course 

 of their relatively slow expansion (diastole) they become 

 filled with cell-sap, which is forced out on the sudden contrac- 

 tion (systole). 



Nothing is certainly known as to the physiological signifi- 

 cance of the pulsating vacuoles. It seems obvious to suggest, 

 as Cohn has done, since in Gonium and Chlamydomonas the 

 contractile vacuole apparently communicates with the per- 

 manent vacuole, that they assist in the distribution of nutri- 

 ment or of oxygen. The fact that pulsating vacuoles have 

 been exclusively found in motile organisms, such as Volvox, 

 Gonium, Eudorina, the plasmodia of Myxomycetes, the zoos- 

 pores of many Algae (Chaetophora, Ulothrix, etc.) and of some 

 Fungi (some sp. of Cystopus, Myxomycetes), seems to suggest, 

 as Engelmann has pointed out, some possible connexion 

 between the vacuoles and locomotion. 



Before entering upon the consideration of the movements 

 of cellular organs, it will be convenient to discuss the relation 

 of the movements already described to external conditions. 

 It was pointed out in a previous lecture (p. 297) that they can 

 only be performed within certain limits of temperature. A 

 second essential condition, in the case, at least, of aerobiotic 

 plants, is the presence of free oxygen; it has been already 

 mentioned (p. 297) that motile anaerobiotic plants continue to 

 move in the absence of free oxygen. 



In the majority of cases, exposure to light is not an essen- 

 tial condition of these protoplasmic movements. Sachs and 

 others have observed, for instance, that the streaming of the 

 protoplasm in plant-cells continues in prolonged darkness, and 

 that it occurs in the cells of etiolated plants. The effect of 



