MOVEMENT OF PROTOPLASM 85 



a limb ; while the co-ordinated movements of the 

 limbs may result in the movement of the body from 

 place to place. 



In the cells of plants the rapid streaming movement 

 of the cytoplasm, which in certain cases may be observed 

 under the microscope to stream round the cell, is perhaps 

 always the result of an injury or other shock : there is 

 no evidence that it occurs normally in the untouched 

 plant. Nevertheless it provides conspicuous proof of 

 the power of plant protoplasm to move comparatively 

 rapidly. There is also plenty of evidence of the power 

 of rapid movement of plant cells under normal conditions, 

 e.g. in the motility of many of the lower unicellular 

 plants and of the reproductive cells in many fixed 

 plants, though these move by means of the contraction 

 of special delicate cytoplasmic processes projecting from 

 the cell body [cilia and flagella) whose rapid beating 

 pulls or pushes the organism through the surrounding 

 liquid. Many unicellular animals also move in this way. 



The commonest form of movement of the protoplasm 

 of plant cells is, however, a slow streaming movement, 

 usually too slow to observe directly, but evidenced by 

 the frequent changes in place of the constituents of the 

 living cell body, such as the nucleus, and in green cells, 

 the chloroplasts. The growth of a plant clearly involves 

 the result of a multitude of slow movements, and all 

 of these, like the much quicker movements of animals, 

 depend upon a constant supply of energy which is 

 derived from the release of potential energy by the 

 breaking down of organic molecules through oxidation. 

 A moment's reflection will convince us of the large 

 amount of energy involved in the building up and 

 raising of the branches of a tree, eventually weighing 

 perhaps many tons, high into the air against the force 



