Ch. Ill, 5] PROTOPLASM 37 



A characteristic feature of the living protoplasm in plant 

 cells is its streaming, manifest by a steady movement of 

 the included particles which obviously are carried along 

 passively by currents of the protoplasm itself. In some 

 cells, especially the very large ones of certain Algae, the 

 streaming is so active, even up to 10 millimeters per minute, 

 that the protoplasm seems literally to rush across the field 

 of a high-power objective, while in others, and especially in 

 young cells completely filled by the protoplasm, special 

 methods are required for its detection ; and all intermediate 

 degrees occur. The streaming is maintained by energy re- 

 leased from food by the protoplasm, and apparently it serves 

 to promote the commingling and transportation of substances 

 throughout the cell. 



Thus it is evident that protoplasm possesses no visible 

 mechanical constitution such as might be anticipated in 

 so remarkable a material. But what is its real ultimate 

 constitution or texture, which cannot be as simple as it looks? 

 The exceptional interest of this problem has stimulated the 

 most profound researches, supported by the most refined 

 methods, but as yet without satisfactory result. It was 

 formerly thought, from the appearance of material which 

 had been killed, stained, and sectioned, that the working 

 protoplasm consists of a tangle of flexible fine fibers holding 

 the food granules and various fluids in their meshwork. 

 Later researches, however, seem to show that it has rather 

 the nature of a foam or emulsion, commonly obscure but 

 demonstrable by special methods, in which small globules 

 of various dimensions and different materials are suspended 

 and held apart by thin films of a certain continuous sub- 

 stance ; while variously intermingled are food granules, and 

 other small bodies of uncertain significance (Fig. 13). Proba- 

 bly the usual ground structure of most protoplasm is thus 

 alveolar, though it develops fibrous elements on occasion. 



Thus the physical structure of protoplasm, in so far as 

 known, gives little clew to the source of its remarkable 



