PROPERTIES OF VEGETABLE PROTOPLASM 347 
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 not clearly understood. 
Certain filamentous Alge, known as the Oscillatoria, 
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. Hach 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 
