MOVEMENT 



stream out from the unicellular body of the radiolaria 

 and thalamophora, thousands of fine particles move 

 about, like promenaders in a street. This movement 

 does not come from the passive granules, but from the 

 active invisible molecules of the plasm, which are always 

 changing their relative positions. Thus also the move- 

 ments of the blood-cells which we can see with the micro- 

 scope in the circulation of a young transparent fish, or 

 in the tail of a frog-larva, are not due to the action of 

 the blood-cells themselves, but to the flow of the blood 

 caused by the beat of the heart. 



An important factor in the life of many organisms, 

 especially the higher plants, is the physical phenomenon 

 called imbibition; it consists in the penetration of water 

 between the molecules of solid bodies (drawn to them 

 by molecular attraction), and the consequent displace- 

 ment of the molecules by the fluid. In this way the 

 volume of the solid body is increased, and movements 

 are produced which may have the appearance of vital 

 processes. The energy of these imbibitional bodies is 

 notoriously very powerful; we can, for instance, split 

 large blocks of stone by the insertion of a piece of wood 

 dipped in water. As the cellulose membrane of plant- 

 cells has this property of imbibition in a high degree 

 (either in the living or the dead cell), the movements it 

 causes are of great physiological importance. This is 

 especially the case when the imbibition of the cell wall 

 is one-sided, and causes a bending of the cell. In con- 

 sequence of the unequal strain in the drying of many 

 fruits, they split open and project their seeds to some 

 distance (as do the poppy, snap-dragon, etc.). The 

 moss-capsules also empty their spores as a result of im- 

 bibition-curving (in the teeth of the openings of the 

 spore-cases). The hygroscopic points of the heron-bill 

 {crodiiim) curl up in the dry state and stretch out when 

 moist; hence they are used as hygrometers in the con- 



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