348 VEGETABLE PHYSIOLOGY 



organisms are conspicuous by their power of locomotion. 

 These are the Diatoms which are so prominent in ponds 

 and sluggish streams. They are unicellular plants of very 

 minute size, each of which consists of a protoplast encased 

 in two silicified shells or valves which fit together very 

 tightly, one overlapping the other by its edges. The cell- 

 wall which forms each valve is strongly impregnated with 

 silica, the latter being deposited in patterns which are 

 often of great regularity and beauty. The plants are not 

 provided with cilia, nor so far as we know are the silicious 

 valves perforated in any way. Each diatom is however 

 capable of effecting a peculiar gliding and very rapid move- 

 ment through the water, the mechanism of which is at 

 present unexplained. 



Certain filamentous Algae, known as the OsciHatoria, 

 also carry out a peculiar movement. They consist of long 

 chains of protoplasts, each separated from its neighbour 

 by a cell-wall, and the whole thread surrounded or coated 

 by a peculiar semi-gelatinous sheath. Each chain is 

 anchored to a substratum of stone or rock at one end, and 

 the free portion is in constant waving or twisting motion 

 to and fro, a movement which is quite independent of 

 currents in the water, being exhibited in the total absence 

 of such disturbance. The movement appears to resemble 

 that of the Diatoms, but its mechanism is at present un- 

 explained. Like the others so far discussed it is one of 

 the features of the life of the organisms, and is carried out 

 by their protoplasm without excitation by an external 

 stimulus. 



In certain organisms of still humbler type another 

 manifestation of the power of contractility can be observed. 

 These are unicellular beings consisting of small unclothed 

 masses of protoplasm. In their substance at some point 

 there may be seen a clear space or vacuole which exhibits 

 a more or less regular pulsation, assuming slowly the 

 appearance of a nearly spherical cavity and then suddenly 

 disappearing, recalling the active contraction of animal 



