MOVEMENT 



Many leaves and flowers hold themselves vertically to 

 the streaming rays of the sun. When darkness comes 

 on they contract, and the calices of the flowers close. 

 Many flowers are open for only a few hours a day. The 

 mechanism of turgescence, which effects these swelling 

 movements, consists in the co-operation of the osmotic 

 pressure of the internal cell-fluid and the elasticity of 

 the strained cell-membrane enclosing the cytoplasm. 

 The strain of the outer cellulose membrane on the plas- 

 matic primordial sac within it grows so much on the 

 accession of osmotically active matter that the internal 

 pressure is equal to several atmospheres, and the elastic 

 strained membrane stretches from ten to twenty per 

 cent. When water is withdrawn again from one of 

 these swollen or turgescent cells, the membrane con- 

 tracts; the cell becomes smaller, and the tissue looser. 

 Other stiriiuli besides light (heat, pressure, electricity) 

 may produce these expansional variations, and, as a 

 consequence of it, certain reflex movements (or para- 

 tonic variational movements). The most striking and 

 familiar examples are the flesh-eating fly-trap {dionwa 

 rmiscipula) and the sensitive plant {mimosa pudicd); 

 their contraction is caused by mechanical stimuli, shak- 

 ing, pressure, or the touching of the leaves. 



Most of the higher animals have the power of free and 

 voluntary locomotion. It is, however, wanting in some 

 of the lower classes, which spend the greater part of 

 their Ufe at the bottom of the water, like plants. Hence 

 these were formerly held to be vegetable — thus the 

 sponges, polyps, and corals among the coelenteria. A 

 number of classes of the coelomaria have also adopted 

 the stationary life, such as the bryozoa and the spiro- 

 branchia among the vermalia, many mussels (oysters, 

 etc.), the actinia among the tunicates, the sea-lilies 

 (crinoidea) among the echinoderms, and even highly 

 organized articulata, such as the tube-worms {tubicolm), 



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